Branchiostoma

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.

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.  

Notochord

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.

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.