The exclusively sea-living class Caudofoveata is among the least known molluscs. Only by the end of the 19th century it became known that they are molluscs; before they had been placed among the holothurians, worm like echinoderms, related to starfish. So were the solenogasters (Solenogastres).
Chaetoderma intermedium, a caudofoveatan.
Picture: Nina Mikkelsen, Bergen University.
Caudofoveatans generally are very small, mostly only few centimetres long. They live in a depth of more than 20 m, where they can appear in a density of up to 4 to 5 individuals per square metre, as far down to the deep sea.
Externally, caudofoveatans resemble worm-like beings. They have no recognizable shell, which is why they are counted among the so-called shell-less molluscs ("Aplacophora"). With a closer look, some characters typical for molluscs can be recognized. Though caudofoveatans do not have a shell, they do have a sturdy exterior tissue, the cuticula. This cuticula is additionally protected by calcareous scales, which are estimated to be a predecessor of the more highly developed molluscs' shell. Chitons (Polyplacophora) also have this cuticula, but only as a rudimentary protection around their sides, the so-called girdle or perinotum. The rest of the molluscs' back is protected by shell plates, that are thought to have evolved from calcareous scales we find today among caudofoveatans.
It is well imaginable that the first molluscs have evolved from worm-like ancestors, such as the caudofoveatans, which lived on the ocean floor or dug in it, and whose sturdy skin was protected by calcareous scales, which in time would change into the astounding shell of most of today's mollusc groups. The worm-like form of today's caudofoveatans, though, is the result of a secondary reduction of the foot, more advanced in this group, than, for example, in the solenogasters.
Head region von Chaetoderma canadense. Picture: R. Robertson;
Source: Woods Hole Oceanographic Institution.
The evolution of the mollusc shell.
Foraminifers, the main food of
scaphopods and caudofoveatans.
At their head end, caudofoveatans have a hard shield studded with sense cells helping the creature's orientation. Caudofoveatans live on the ocean floor, where they look for food, either crawling or digging. On the one hand they are detritus eaters, so they live on decaying organic matter, or they feed on monocellular organisms, such as foraminifers and diatoms (silicate algae).
As in all molluscs, also in caudofoveatans, feeding takes place with the help of a rasp tongue, the radula, which may have as many as 1,000 toothlets. Caudofoveatans, though, do not yet possess the ribbon-like radula of higher molluscs; theirs grows from the oesophagus wall. So the radula is a very old character, which seems to have appeared with the very first molluscs long ago. Among caudofoveatans, the radula is quite variable, comparable to the radula of a gastropod. In that regard, caudofoveatans are different from scaphopods, similar in their digging way of life, and chitons (Polyplacophora).
As do other molluscs, caudofoveatans also have a pallial cavity, only it is very small and located at the body's end. In the pallial cavity, there are the paired comb-gills or ctenidia. The gonad, the sexual organ, is located on the dorsal side and from it a efferent duct leads to the pericardium (the heart bag) and from there several ducts open into the pallial cavity, also a very ancient character. The dorsal gonad is also present in chitons (Polyplacophora), so it is a character which both groups have in common. Caudofoveatans have separate sexes, so there are males and females, fertilisation takes place externally in the water. As it does among the solenogasters, larval development in caudofoveatans takes place passing a planktontic trochophora stage.
The neural system of the Caudofoveata is simple and built like a rope ladder. There is, though, a recognizable cerebral ganglion to provide the cerebral shield with neurons.
Salvini-Plawen, L.; Mizzaro-Wimmer, M.: "Praktische Malakologie - Beiträge zur vergleichend anatomischen Bearbeitung der Mollusken", Vienna 2001.