|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.