Sunday, March 30, 2014

Good Morning Jack...

Letter to my Son
Sunday, 30 March

We're all somebody's meal one day

Good Morning Jack…

I was caught in the rain yesterday while on a long walk.  My clothes were drenched but it didn’t matter.  It’s spring and the rain wasn’t cold.  No more freezing nights.  Soon we will switch from heater to air conditioner here in North Carolina.  But for a short period there won’t be need for either.  Today is right for opening windows and letting the breeze sweep all the winter’s cabin fever from the house. 

Friday a woodpecker came knocking.  He was right above the window to my room.  They really are quite loud.  You’d think they were driving nails with their bill.  How does their brain take all that pounding?  Terns that spend their life diving into water after fish have a similar problem.  Several times each day they collapse their wings and make a high dive attack at their intended meal.  I’m told smacking your head repeatedly does take its toll and they aren’t likely to live more than three years.  What do you think happens?  Do they slowly knock themselves senseless to the point of senility?  Maybe their vision becomes blurred or they start seeing double.  It’s not like they can go to an optometrist and get fitted for corrective lenses.  Every head throbbing dive becomes a lucky miss for the fish.  Eventually it gets to be too much.  How does a bird process the headaches and all this frustration?  Don’t forget their nagging hunger pangs.  Then there are the youngsters.  You’re no longer part of a team providing food for a hungry nest.  Your spouse is left with all the work.  It just won’t do.  Wanted:  healthy tern to replace the worn out one seen sitting lethargic by the water’s edge.  Life is for the young.  Animals in nature don’t enjoy their golden years.  There aren’t any to enjoy.  Your sorry carcass simply becomes someone else’s nutritious meal.  It’s probably not long in coming, either. 

Most every warm-blooded animal gets hungry on a daily basis – birds especially.  They have to fly to stay alive and they don’t have the luxury of carrying around any body fat of consequence.  You ever see a robin with love handles?  If a bird on a limb looks plump you can be guaranteed it is only because they’ve fluffed up their down for warmth. 

So, yes, in the dog-eat-dog world of Mother Nature there is no need for hiring a janitorial service.  If you’re that worn out tern resting at the water’s edge and you hear “Clean up on Aisle 7” – you can rest assured they are talking about you.  The best you can hope for is a humane death of having some ravished predator first bite into the back of your neck.  Lights out. 

If we’re open-minded about the matter we should probably feel gratified that our spent selves provide a meal for more than one worthy individual.  Usually there’s a vertebrate to provide the ceremonial coup de grace.  But they rarely pick your bones clean.  There’s too little left to make it worth the effort of an animal carrying around a large stomach.  Now it’s a race among the insects.  Flies are quick to sense the aroma of your decay and gladly lay their young amidst your tissue.  Of course, you can never count out the energetic search of the enterprising ants.  Now they truly will pick your bones clean.  How many times have you witnessed a parade of these busy-bodies streaming into the eye socket of some dearly departed animal?  By the end of the day, two at the most, every sinus and cranial cavity becomes remarkably spic and span. 

I suppose I’m still a bit touchy about being eaten.  I can’t grasp the aesthetic beauty of being systematically torn limb from limb and devoured.  If that isn’t bad enough then I have to further endure the humiliation of being defecated.  Now doesn’t that make for a classic before and after picture comparison?  No, I much prefer being consumed by flame.  You can scatter my ashes about the base of a desert mesquite bush.  It’s so much more discrete.  They have such fastidious manners when it comes to feeding. 


Thursday, March 27, 2014


Branchiostoma is commonly called a lancelet

Branchiostoma, formerly referred to as amphioxus, is the living invertebrate chordate that most closely resembles vertebrates.  The ancestors to the first vertebrates may well have been similar to this animal.  The animal is most commonly referred to as the lancelet, undoubtedly because of its long pointed, cigar-shaped, appearance.  These small, translucent animals are one to several inches (up to 50 mm) in length and inhabit the coastline of marine waters.  Their notochord, the most fundamental characteristic of chordates, is unusual because it extends well into the animal’s head.  This is believed to be an adaptation better enabling this animal to burrow in the sand where it spends most of its life.  Despite its appearance the lancelet lives a sedentary life with only its head protruding from the soft sediment that is its home. 

Head shows tentacles around mouth to help feeding

Branchiostoma is a filter feeder much like another protochordate, or non-vertebrate chordate, the tunicates, or sea squirts.  Food particles are swept in with the sea water that enters the mouth.  The water continues into the pharynx, or branchial basket, which filters out the minute bits of food.  The water is strained through the pharyngeal slits, and then enters into an atrium where it is expelled to the outside through the atriopore.  The remaining food particles are snared by a mucous that is secreted from a gland called the endostyle.  The endostyle is located beneath the pharynx and it is this gland that serves as the basis for the thyroid that develops later in vertebrate animals.  The beating of cilia forms the mucous into a food band that gradually moves up the pharynx and into the intestine for the process of digestion. 

Lancelets spend most of their life burrowed

While Branchiostoma lives mostly a sedentary life its body shape and musculature give it the appearance of an active swimmer.  Its segmented musculature, or myotomes, enables it to disperse from its point of birth as well as provide the propulsion needed for burrowing.  Movement is accomplished by waves of muscle contractions that alternate from side to side and extend the length of the body from the head to the tail.  The resulting lateral undulations propel the lancelet forward. 

The notochord extends the full length of the head

Lancelets’ musculature, notochord, dorsal nerve tube, pharyngeal slits and a circulatory system are all characteristics it shares with vertebrates.  Its body plan probably resembles that of the most primitive of vertebrates but it lacks a number of characteristics distinctive of the modern vertebrate.  Its circulatory system is not closed.  Arteries and veins are linked by sinuses.  There is no heart or capillaries.  There are no blood cells.  Oxygen is transported in a solution containing no oxygen-carrying pigment.  There are no gills.  The entire body surface is sufficient for the exchange of respiratory gases.  The animal has bilateral symmetry but there are no true paired fins.  Instead, Branchiostoma has ventrolateral tissue folds that extend from the pharynx to the atriopore. 

Segmented musculature is like that of vertebrates

There is little fossil record of the cephalochordates that include Branchiostoma and the ones discovered only superficially resemble the modern lancelet.  Their soft body makes fossilization extremely rare.  What does exist from the early Cambrian period of more than 500 million years ago reveals a simple, bilaterally symmetrical animal that could conceivably provide the basis for the lineage that arrives at the modern lancelet as well as the lineage the developed into the diverging forms of modern-day vertebrates.

Tuesday, March 25, 2014

Dorsal Hollow Nerve Cord

Nerve Cord foundation for nervous system

The dorsal hollow nerve cord is fundamental to all vertebrates and it serves as the basis for their elaborate central nervous system.  This nerve cord extends beyond animals with vertebrae to also include other chordates in which the vertebrates represent the major part.  Among these animals are the fish-like lancelets, or amphioxus, and tunicates.  The invertebrate tunicates, or sea squirts, seems to be a highly unlikely organism to have a dorsal hollow nerve cord as its sedentary lifestyle and primitive body plan appears to have no need for such a sophisticated nervous system arrangement.  In fact the adult has no such structure and the nerve cord appears only in its mobile larval form.  The lancelet retains its nerve cord but its anterior end does not elaborate into anything one might consider a brain and its head has only rudimentary, unpaired sense receptors. 

Embryo with early neurulation 

The nervous system is made up of cells that give life awareness of its surroundings and, in its highest implementation, a consciousness of the self and a sense of wonder and perspective of the self within all existence.  Imagine that.  Cells are working together to produce the power of thought.  This alone makes creation of nerve cells the most extraordinary development of advanced life forms. 

Development of the nerve cord involves a complex choreography of various types of cells moving with synchronization in both time and space.  It begins at a very early stage of the embryo, during the blastula, when cells first begin to divide and differentiate into what will become three distinct layers of cells.  The outermost layer, the ectoderm, proceeds to form the skin, anterior and posterior parts of the digestive tract as well as much of the nervous system, including the eyes and ears.  The innermost layer, the endoderm, provides lining for the gut and the glands associated with the digestive tract.  The respiratory surfaces of vertebrates also originate from endoderm.  The last of the three layers to usually differentiate is the mesoderm, or middle layer.  Products of mesoderm include the muscles, skeleton, connective tissue and the circulatory and urogenital systems. 

Nerve cord among key chordate characteristics

Neurulation begins when mesodermal cells, called chordamesoderm, collect to form the notochord which becomes the embryo’s body axis.  Presence of the chordamesoderm induces the ectoderm overlying the notochord to develop two longitudinal folds, creating a mid-dorsal furrow between them.  The crests of the two folds grow towards one another, forcing the furrow deeper into the dorsal mesoderm that lies adjacent to the notochord.  These neural folds fuse together to make a tube of isolated ectoderm beneath the surface of the embryo.  This neural tube becomes the basis for the central nervous system. 

During the formation of the neural tube within the embryo of vertebrates another group of cells differentiate themselves from the ectoderm.  Arising in the area between the developing neural tube and the closing ectoderm overhead is a distinct group called neural crest cells.  These cells have great evolutionary importance because they are responsible for the creation of most every characteristic that sets vertebrates apart from all other organisms.  They disperse laterally and ventrally from their point of origin to settle and differentiate into a variety of forms throughout the body.  These migrating neural crest cells become the basis for most of the peripheral nervous system.  They form the autonomic system and several endocrine glands.  They are responsible for much of the head’s skeleton and connective tissue as well as other elements that make up the nervous system.

Anterior nerve cord elaborates into vertebrate brain

A nervous system has developed among insects and other invertebrates but no organism comes close to the refinement of its abilities to comprehend its surroundings and provide reasoned solutions to confronted problems like the power of the nervous systems exhibited among the higher vertebrates.

Sunday, March 23, 2014

Good Morning Jessicca...

Letter to my Daughter
Sunday, 23 March

Calling out cranky behavior

Good Morning Jessicca…

Finally, another spring!  The dead return to the living, so to speak.  Don’t think too hard on that thought.  The sun gives us a longer, more generous shine.  The earth warms.  There are fewer shivers in me timbers on these days.  Feet no longer freeze at night.  Flowers bloom.  Bees buzz and everyone has a poetic verse come to mind.  Well put a bee in your bonnet and may the moon make you think of a big pizza pie.  Yeah, that’s amour, eh?

Let’s make a bountiful Easter hat big enough to feed a family of six.  I’m starving.  Let’s make the hat all about appetite.  How do you spell mozzarella?  Thank God for ‘spell check’.  We need plenty of gooey cheese to drip in long strands over the brim.  I’m partial to pepperoni.  It makes for a good strong color against the rich, bubbling cheese.  We definitely need plenty of pepperoni.  So much pepperoni, in fact, anyone can smell its oven roasted aroma with only the use of their eyes.  You know, I almost want Cheetos sprinkled over my hat.  No, bad idea.  Grilled onion sounds better.  My mouth waters.  Now that’s a hat too splendid to wear.  I did say the size that would serve six, right?  I think maybe six is the new two.  I really am very hungry but I will share it with you. 

Hats smelling this good are meant to be eaten.  We need to accessorize our hat with a pitcher of something frosty cold.  Let’s try it with a movie, as well.  How do you feel about watching people swept up into a world where fists smash bricks and psychic beams send cars flying through the air?  Small moments of mortal life are reserved for bathroom breaks and quick runs for more Doritos. 

We’re all caught up into our own titanic life or death struggle.  Just beyond that innocuous front door is a raging battle to hold back the forces of pure evil.  Go to the window.  Now!  See there… clouds billowing up over the horizon – it’s global, it’s melodrama, it’s black and white, just and unjust, friend versus foe.  Are you prepared?  OK, stand back.  There are others here who wish to follow us into action.  Hallelujah.  They are your brothers.  They need no introduction.

First things first:  the pizza’s gone.  I’m sorry.  I should have ordered more.  This is no time for tears, though.  Come.  We've work to do.  Follow me!

Pretend all kinds of superhero mayhem have just occurred and we've returned victorious, now, to our ultra-secret Fortress of Superheroes for lunch.  I’m famished.  Storm Woman you have the Grilled Chicken Salad with low fat Blue Cheese Dressing.  Colossal Boy has a double-decker cheeseburger, curly fries and a frosty Dew.  Metallica Mind I believe you ordered two Patty Melts with an Oreo Blizzard.  Ultimate Robot has, of course, the Greased Monkey slathered with extra Virgin Oil.  I’m having the peanut butter and jelly sandwich with toasted sour dough bread and a box of Raisonettes.  Dig in, everyone.

Where were we?  Yes, I believe we were discussing the latest diabolical plot of the evil genius – Professor Death Star.  It appears he’s developed a new ragweed allergy that renders useless any over-the-counter remedy, even maximum strength potions.  I've never suffered such miserable sinus headache and drainage.  This is only the Ides of March and the pollen season hasn't even yet begun.  I agree with Storm Woman.  This calls for an extra-large tissue dispenser on all our utility belts.  Good thinking Aurora Mom.  Be sure we all schedule a daily nap.  Combating mortal evil requires we have plenty of rest.  There’s no room for cranky behavior here.  I suggest we also send a super tweet to all our superhero followers.  In their own modest way they too can benefit from all these sensible suggestions for fighting this allergy pestilence.  Good.  It’s settled, then.  We’ll all meet by the flag pole following our naps.  Remember… capes are mandatory.


Thursday, March 20, 2014

Pharyngeal Slits

Pharyngeal slit arrangements of invertebrate chordates

The pharyngeal slits are among the four defining characteristics of chordates and make their appearance in all vertebrates at some point in their life.  They were originally part of an invertebrate suspension-feeding device.  This mechanism is well illustrated in modern animals by the adult tunicates, or sea squirts, and amphioxus, an invertebrate chordate that appears much like a present day fish.  These animals are all filter feeders that rely on straining minute food particles from waters that pass through holes in the pharynx.

A shark's pharyngeal gill slits

The pharynx is a cavity that exists immediately behind the mouth.  In filter-feeding chordates and vertebrate fish the pharynx is perforated with a variable number of holes that allow a current of water to pass through the mouth and into the pharynx, then, out through its holes or pharyngeal slits.  Small, hair-like structures called cilia create a beating motion in invertebrate chordates that induces this flow of water.  The pharynx itself is lined with mucus that is used to snare the suspended food particles.  Cilia complete the feeding process by moving the food-enriched mucus to the animal’s esophagus and then digestion in its gut.  Vertebrate fish subsist on larger prey and rely on muscular action, not the rhythmic beating of cilia, to produce the current that enters by way of their mouth and exiting through their pharyngeal perforations.  In this case the water supplies the animal with oxygen and not the nutrition from food.

Embryonic land vertebrates have unperforated pouches

This perforated pharynx feeding mechanism of ancient chordates provided the framework for the evolution of subsequent features that include the pharyngeal muscular pump, internal gills and vertebrate jaws.  Along the arches that separate the individual pharyngeal slits began the development of tiny plates or folds of tissue.  Over time this tissue became increasingly vascularized, harboring beds of capillaries that were rich with blood.  The role of capturing food along this tissue became secondary to providing respiration for these increasingly large, and active, animals.  The larger body size and the higher metabolic rate required for an active predator meant the need for respiratory efficiency beyond that of small, sedentary organisms.  For the first time the term pharyngeal gill slits could be accurately applied to these specialized structures of vertebrate fish.  Muscular pharyngeal pumping forced water over these gills, enabling oxygen to be absorbed by the animal while carbon dioxide would be diffused from the gills into the passing current. 

Pharyngeal arches have new roles rather than disappear

While pharyngeal slits persist into adulthood with bony fish as gills, their embryonic form in most land animals is overgrown and no longer appear in their initial form or role.  The pockets in the embryonic pharyngeal cavity of vertebrate tetrapods never break through to become slits.  Instead they remain grooves, or pouches, that give rise to other structures, including the Eustachian tube, middle ear cavity, tonsils, parathyroid glands and other tissues associated with the lower jaw and neck. 

The fact the each cell in the body of an organism carries the animal’s complete DNA blueprint undoubtedly contributes to the species’ ability to differentiate as needs change over time.  Structural tissue has the potential to become respiratory tissue which, in turn, may evolve into a hormonal producing gland.  This statement itself is probably an error in its simplification but both fossil and genetic evidence clearly illustrates the transformational talent that life forms exhibit as they change to meet the new challenges presented over geologic time.  

Tuesday, March 18, 2014


Lamprey notochord and vacuolated core cells

The notochord, a slender elastic-like rod, is one of four biological features that draw together a wide range of animals into a single grouping named Chordata.  Along with the notochord, the dorsal nerve cord, pharyngeal slits and postanal tail are characteristics shared by an assemblage that includes both humans and the sac-like sea squirt.  Vertebrates make up the vast majority of the animals represented but the larval form of the marine sea squirt gives them admission to this distinguished club, as well. 

45 hour old chick embryo with notochord

The notochord is a hydrostatic organ with a tough outer wall enclosing a fluid core.  This gives it lateral elasticity while enabling it to resist any axial compression.  Anchored to this rod, that extends nearly the length of the organism, is a series of segmented muscles used, in most instances, to give the animal the means of propulsion through the water.  The contraction of muscles on one side and then to the next provides alternating lateral pressure against the surrounding substrate.  The resulting undulating motion propels the animal forward.  Once the muscles relax after contracting on one side of the body the springy notochord acts to straighten the body out.  The notochord acts as the antagonist against the muscles’ action, enhancing the sweeping of the tail from side to side. 

Lamprey notochord extending beneath brain

The hydrostatic nature of the notochord prevents the compression of the animal’s axis which would severely hinder its ability to swim.  This pressure is provided by fluid residing between the notochord’s core cells or by core cells swollen with vacuoles containing fluid.  These vacuolated cells are wrapped tight within a sheath of tough, fibrous tissue.  Under these conditions the inner fluid is held fixed, unable to flow. 

Zebrafish embryo with notochord, segmented muscles

The notochord may persist in more primitive chordates but in the case of bony fish and terrestrial vertebrates this rod is replaced by the vertebral column.  In these instances the notochord appears as a structure used as a scaffold around which the embryonic body can grow.  It makes its appearance early when the mesodermal layers at the dorsal midline differentiates into the chordamesoderm tissue.  This gives rise to the notochord as well as further stimulating the differentiation of the overlying ectoderm into producing the central nervous system.   It is consequently above the body’s main central cavity, or coelom, and beneath the dorsal nerve cord.

Human vertebrae with notochord derived discs

The notochord does not necessarily disappear.  In adult mammals it has transformed into a series of intervertebral disks.  These form circular pads that lie between the successive vertebrae.  Each pad is a fibrocartilage tissue that encloses a gel-like core, called the nucleus pulposus, providing a cushion between the connected bony vertebrae.  If you’ve ever suffered a slipped, ruptured or crushed disc you know how important these structures can be to your general well-being and a healthy frame of mind.

Sunday, March 16, 2014

Good Morning Justin...

Letter to my Son
Sunday, 16 March

A thousand words or more

Good Morning Justin…

The Bradford Pear are in full bloom everywhere.  The tree seems to be in just about every other yard around here so, yes, they are blooming everywhere.  I just noticed it this morning when I took a long walk to Steve’s Garage over on Dale Earnhardt Boulevard.  The funny thing is I took the same walk yesterday (twice!) and I hadn't noticed a single tree in bloom.  Sure, maybe I was preoccupied.  But I had my binoculars with me so it’s not like I wasn’t looking around.  Anyway, the point is these trees bloom all at the same time.  Come March 15th and they all burst full with popcorn blooms.

That’s something I should put on Facebook.  That’s where we all go to share these days, isn’t it?  What is the point of Facebook if it isn’t to share information?  I think that’s about all we do there.  Exchanging information is so a part of who we are that we take it for granted that this is something we would want to do.  Twitter is more of the same, isn’t it?  The forum is a bit different but it’s where you go to find out about what someone did or thought.  Newt Gingrich, a politician, tweeted what kind of sandwich he and his daughter had for lunch one day.  I suppose if you’re sort of a celebrity this is the kind of thing some people who follow you are curious to know.  For some reason I felt it important you know I saw Bradford Pear blooming. 

Can we step outside of ourselves far enough to find amazement in our own human behavior… even if it seems trivial?  Talking among ourselves about what we saw, what we thought, what we felt – are all not trivial matters.  What greater foundation to our success as a species is there than our instinctive desire to communicate with one another?  It starts with our face.  What a window into our thoughts it can be.  It takes effort and some talent to present to the world a poker face when we are charged with emotion.  It’s the money card to what makes for a great actor.  You’ve been revealed as perpetrator of some great unforgivable sin.  Words escape you.  The director wants a close up of your face that will say it all. 

It’s not just the face that has impact in communicating our feelings.  Once again the director feels words are cheap.  You’re being left by the one you love for another.  You’ve made a life together.  Now you are stunned, crushed, as the dearest person to you walks out of your life forever.  You stand mute following with your eyes as your lover drives away.  She looks back in the rear view mirror to see the figure of devastation.  The body language says it all. 

Of course, words are our great accomplishment.  We’ve created a language not simply of nouns and verbs to express our questions and commands.  We have subtle minds and we often require precision in how we speak.  Grammar gives our language a structure to minimize the possibility that we be misunderstood.  Voicing our thoughts with words requires the ear.  But our eyes are so much better suited to receiving information.  Now we make curious squiggles to represent words for the eye to digest.  We are rich in the ability to share information.  We tell stories and they give our recounting of facts great emotional weight.  We explain our thinking one to another.  Sometimes our need for precision is so great that we rely only on numbers to convey our reasoning behind abstract conceptions.  There are times when what we have to convey requires the purity of paint or dynamics of music to convey a finely tuned emotional sense – something that exists only when carefully balanced.  A shift of feeling either way and our expression topples into something crudely hammered, as though with nails that too easily bend to each blow. 

Picture in your mind a morning sun hidden by falling snow.  This is what human words do.


Friday, March 14, 2014


Filter feeding Ostracoderms with head shields

Ostracoderms are among the earliest vertebrate fossils going back over 500 million years to the late Cambrian period.  They are a grouping of convenience and do not represent a natural evolutionary assemblage.  For the most part the term Ostracoderm describes several extinct groups of armored, but jawless, fish.  They are generally small, ranging up to only a few centimeters, with some exceptions.  They have no paired fins so their swimming ability would be somewhat hampered by a lack of stability.  Much like Archaeopteryx represented a very early effort at winged flight despite real limitations the earliest swimming vertebrates worked with similar structural handicaps.  Despite this, these animals succeeded.  The reason being is that they were the pioneers of a new style of life and they had no other competition during their initial radiation throughout the watery environment. 

Pteraspids displaying dorsal spines on armor

The bony scales of this class of animal gave them the name Ostracoderm, which means “shell skins”.  Besides scales many of these animals also had heavy armor, particularly about the head.  This armor was made up of three principle layers – an outer layer which consisted of dentine with possible enamel projections; a middle layer of bone that was riddled with channels inhabited by blood vessels and, possibly, sensory pits; the innermost layer also bone, but with few vascular channels, and lamellar, layered like an onion.  The most primitive vertebrates would be generally more heavily armored but, over the millions of years, their descendants would optimize the virtue of strength with the burden of weight. 

While the armor of ostracoderms contained elements of bone the animal’s internal skeleton was all cartilage much as are the skeletons of modern sharks.   Armor would protect the animal’s brain and well as its delicate gills.  The gills were not slits like most fish today but existed as filaments arranged over the surface of a pouchlike chamber that exited to the body’s surface through a pore.  These gill chambers could be highly variable in number, depending on the type of ostracoderm, ranging from five to fifteen on each side of the animal. 

Anaspids were without head armor

The earliest ostracoderms were undoubtedly filter feeders, as their mouth was very small and they had virtually no oral cavity.  There was a large pharynx, though, and mucus would cover the gill bars, trapping the microscopic food particles.  Cilia would move the food-enriched mucus to the intestine, completing the feeding process.  Over time most ostracoderm variations developed larger mouths and oral cavities that came with a rasping organ that served much like a tongue.  Despite obtaining these features ostracoderms, such as anaspids and cephalaspids, were limited to nibbling on smaller animals because they lacked jaws and, presumably, anything we might consider teeth. 

Earliest Placoderms had jaws but no paired appendages

The ostracoderms flourished for more than 100 million years before their extinction in the late Devonian period.  By that time numerous forms of jawed fish were beginning to dominate the environments of both fresh and marine waters.  Jaws, and the teeth that eventually came with them, opened up the diet possibilities of animals.  Jawed fish would be able to successfully radiate into all available niches and eventually outcompete the more ancient body-plan of ostracoderm.  The appearance of paired lateral appendages in the form of fins added immensely to the fishes’ ability to swim and maneuver.  Given the fact that placoderms, or jawed fish, were around at least fifty million years prior to the extinction of ostracoderms makes the length of reign of these jawless vertebrates all the more remarkable.  

Wednesday, March 12, 2014

Ammocoete and Primitive Vertebrates

Ammocoete with gill-like pores and hooded mouth

Ammocoetes are the long-lived larva of the jawless lamprey, a parasitic vertebrate in its adult form.  The larval lamprey lives as a mostly sedentary filter-feeder in the soft sediment of freshwater streams.  It can live in this larval stage for up to seven years before its metamorphosis into a marine or freshwater adult.  The adult lamprey usually lives one, possibly two, years before it reproduces.  In the case of the North American brook lamprey the animal breeds in the spring following its metamorphosis and never feeds, so it is not parasitic.  In any event the female and male adult animals die once the eggs are laid and fertilized.

Adult Lamprey with gill-like pores and toothed mouth

North America has 41 known species of lamprey of which 19 are strictly freshwater.  It has the appearance of a primitive looking eel.  The lamprey has no jaws, no scales, no paired fins like other fish and it has no bone.  It does have teeth on its tongue that it uses to rasp a hole in the side of large fish to which it clings and feeds.  The lamprey has all the features needed to classify it as a vertebrate and it belongs in the class of animals known as Agnatha.  Except for lamprey and hagfish all the many representatives of this jawless class of vertebrates have been extinct for more than 200 million years.  These ancient agnathans are believed to be the first known vertebrates to evolve.

Lamprey has no paired appendages

Biologists concerned with how these first vertebrates lived have focused their interest on the ammocoete, the larval form of the lamprey.  Even though the adult lamprey retains most of the characteristics of its larval stage, it is the ammocoete that is believed to most closely resemble the body plan of the earliest agnathans.  The ammocoete has a long, slender body with an oral hood that surrounds its mouth.  The animal feeds on micro-organisms in the water it takes in by creating a current through a muscular pumping action that is much like modern fish.  The base of its pharynx has what is called an endostyle, a glandular groove that produces mucus – trapping the food particles and passing them directly to the intestine through the patterned beating of cilia

Lamprey's mouth is a suction cup with teeth

There is a rodlike structure, the notochord, which extends the length of the body and serves as the skeletal axis from which muscles are attached.  The skeleton of lamprey is cartilaginous.  There is a dorsal nerve cord above the digestive tract from which nerves extend to enervate the animal’s segmented muscles enabling it to propel itself through the water using an undulating motion.  The anterior portion of the nerve cord is enlarged to create the animal’s brain.  Two eyes are present but they lie beneath layers of skin and will not emerge until the adult stage.  There is one median nostril, auditory vesicles as well as both the thyroid and pituitary glands.  There are also seven pair of pharyngeal pouches with pharyngeal bars separating these perforated pouches and act in the manner of gills on modern fish. 

Lamprey cling to fish like giant leeches 

Ammocoete has kidneys and a liver with a gall bladder.  It has pancreatic tissue but no distinct pancreatic gland.  There is also a closed circulatory system with a two-chambered heart – containing an atrium and a ventricle.  The atrium receives blood from the veins while the ventricle pumps blood into the arteries. 

The hypothesis that the earliest vertebrates were likely to have a body plan such as this has to have been drawn from inductive reasoning as the fossil record gives little evidence as to the nature of an animal’s soft, internal tissue.  Comparative anatomy of contemporary species is one source and DNA mapping increases in its influence as genetic coding becomes better known and understood. 

Monday, March 10, 2014

Chordata: Link to Invertebrates

Tunicate larva and tadpole

The major grouping of animals, or phylum, known as Chordata is of particular interest because it contains all vertebrate animals (subphylum Vertebrata) as well as provides a historic relationship with a number of organisms, like starfish, that show no identifiable similarities with mammals seen in the zoo.  What ancestor we have in common with this invertebrate would have to go back many millions of years.  Starfish belong to the phylum Echinodermata.  This grouping is made up of sea urchins, sea cucumbers and other marine animals in addition to the starfish.  Their link to us probably goes back well over five hundred million years, to the early Paleozoic era – near the beginning of fossils that have been discovered with the unaided eye. 

There is no clue about vertebrate origins when you examine an adult echinoderm.  Their morphology lacks any of the characteristics we associate with modern vertebrates – such as a brain and an internal skeleton.  The relationship with vertebrate ancestors is found in the larval stage of animals similar to these.  The larva of the sea squirt, the tunicates, has some of the hallmark characteristics found in Chordates – ancestors to the vertebrates.  These include a notochord (which serves as a backbone), a dorsal nerve cord and segmented muscles – all characteristics found in today’s vertebrate animals. 

Adult sea squirts

How juvenile traits are retained by adult descendants is explained by the process called paedomorphosis.  For chordates to descend from the larval form of some ancestral echinoderm would require the larva’s reproductive organs to mature prior to reaching the adult stage.  Descendants of this process might retain these larval characteristics into adulthood if they prove beneficial to the animal’s survival. 

The notochord is a flexible, rodlike structure that extends the length of the body providing the animal an axis for muscle attachment and giving the animal the undulating movement needed for propulsion through water.   For vertebrates the notochord appears during embryonic development and becomes the basis for the vertebrae. 

The dorsal nerve cord enables the development of a central nervous system and the enlargement of the anterior end into what becomes the organism’s brain.  This allows for a more sophisticated body plan based on greater awareness of one’s surroundings and the means to quickly move to more suitable locations and to pursue prey. 

Segmented musculature controlled by a centralized nervous system improves coordination and to enable a response time rapid enough to effectively pursue prey or to avoid the lunge of a predator.  These elements, found in the ancient fossils of the first chordates, are elaborated upon over time to include a postanal tail – a muscular appendage extending beyond the anus that significantly improves the animal’s means of propulsion.

The earliest chordates undoubtedly had pharyngeal pouches that were perforated and would be used for filter feeding.  These slits enabling water to pass through would later be the basis for respiration, gills that enabled the exchange of gases between the animal and the water.  Embryonic pharyngeal pouches give rise to the Eustachian tube, the middle ear and other devices in land-based vertebrates. 

X-Ray Tetra reveals vertebra

Other chordate characteristics that would be increasingly elaborated upon over time would include an endoskeleton, paired appendages, a complete digestive tract and a ventral heart with a closed blood system to go with it.  The body plan worked because it proved to be extremely adaptable and the resulting vertebrates were able to widely diversify in form and function to fit the demands of most of the world’s habitats.   

Sunday, March 9, 2014

Good Morning Jacob...

Letter to my Son
Sunday, 9 March

I think I can . . .

Good Morning Jacob…

Life is often experienced with powerful emotions pulling you in one direction, then another, when you are a young artist.  You rarely pause long enough to appreciate knowing a middle ground.  You love intensely.  You hate bitterly.  You fall for beauty.  You face, stupefied, the bleak.  And since you are an artist you attempt to depict it all, in your own fashion.  There’s the glimmer of sun peering beneath the window shade drawn tightly down.  The room is smudged in unsettled blue.  The door is the color of a promise – soon it will open and she will be there.  In pictures of the heart, promises need not be kept.  I’m glad you are an artist but life is a precarious moth swooning round the candle flame. 

I’m drawing on my own ambivalence to create a picture headed nowhere.  That’s the theme:  ambivalence – intensely contrasting.  How does mine hold up to expression?  What degree of passion is revealed?  What imagery from life will be depicted and in what fashion?  What sort of caring results from my own ambivalence about most everything?

It was a cold rain for Friday’s funeral.  Lewis was the man next door.  He had a small, ink black dog with big bug eyes and incredibly long thin legs.  She looked like a whimsical character out of an old Betty Boop cartoon.  She found my very presence anywhere in her vicinity annoying and she would always come yapping after me.  Lewis would yell, “Miss Dale!” to call her off.  Lewis kept his red Ford Ranger pickup clean and usually parked it facing the street under his carport.  He had worked at least the last ten years at Sam’s Club when he died.  There was a stunning splash of flowers atop his casket when I went to see him at Whitley’s Funeral Home.  The suit he wore was perfectly tailored and he had the appearance of a highly regarded member of the business community.  His nails were expertly manicured and placed in lasting repose.  He looked dignified but gracious.  His wife, Kay, had slipped a small toy car, precisely centered, in the breast pocket of his suit jacket.  It had the number 3 on the car’s doors… the number of Kannapolis, North Carolina’s home town hero – Dale Earnhardt.  Lewis was a lifelong NASCAR fan and he was always true to Dale and his son, Junior – number ‘88’.  The toy car was the one thing his wife chose for him to take with him to his grave.

The skies cleared the following day, Saturday, and temperatures rose into the sixties.  Birds everywhere made their appearance.  Maybe they sense the onset of spring.  The flocks of robin have been breaking off into couples and disappearing into the surrounding trees and brush.  The male cardinals are all dressed up in stunning new vermilion feathers to dazzle the local females.  Soon the ladies will be working dawn to dusk gathering material to build a nest, laying eggs, protecting their eggs, hatching them and then flying about the landscape gathering food for incessantly demanding youngsters, mouth wide agape and squawking.  Such is the price paid for giving in to the male’s display of love. 

I think I see bits of green breaking the ground into sunlight here and there.  A Carolina Wren searches eagerly for its first meal of insects, the precocious ones that will hatch and bravely step forward.  High up in the still bare branches of trees are large clumps of nesting material where soon the squirrels will give rise to another generation of families.  It will be sunny and clear once again today and I am set, ticket holder for a new season of theater brought to life by the lengthening days and renewed warmth of an ever climbing sun.