Caudofoveata

The caudofoveata (Caudofoveata, from the Latin cauda, tail and fovea, fossa, plus the ending of neuter plural participle -ata), chaetoderms or chaetodermomorphs (with setae on the dermis) are a class of the phylum Mollusca. For years it formed, together with Solenogastres, the Aplacophora class, considered today as paraphyletic.

Only 141 species are known, all of which have burrowing habits (they make a gallery and live upside down). They are vermiform animals without a shell, although their outer part (the mantle) is formed by a thickened chitinous cuticle with calcareous and aragonitic spicules. They are small in size, although the largest can reach 14-15 cm.

General characteristics

Caudofoveates, roughly speaking, externally resemble the hypothetical ancestral mollusc of Salvini-Plawen and internally the hypothetical ancestral mollusc of Yonge.

External Anatomy

Caudofoveates lack eyes, tentacles and feet (since they are not crawlers but burrowers, living in vertical tunnels at great depths in the seabed) and have poorly developed heads. They have a vermiform appearance (like worms) and are infaunal (live under the substrate). They present in their anterior region, on the head, a characteristic perioral or suboral foot shield. This is composed of a hardened cuticle that serves as a burrowing organ and as a sensory organ. At the posterior end is the paleal cavity (as in the hypothetical Salvini-Plawen mollusk), with a bell morphology and formed by the folds of the mantle where a pair of bipectinate ctenidia (gills) are housed and where the anus opens; this cavity is the fovea, from which the name of the group comes.

Digestive system and nutrition

The digestive system of the caudofovate is similar to that described, in general terms, in the hypothetical Yonge mollusk. They have an oral shield, a mouth with a reduced distichous radula (with two rows of teeth) with its muscles, as well as salivary glands that flow into the esophagus. They have a stomach that has a very large ventral blind sac, into which food does not enter, but rather secretes digestive substances. Behind the stomach is an intestine that ends through the anus in the paleal cavity.

They are sedimentivores (they feed on sediments such as foraminifera and diatoms), detritivores, and endofaunal microphages.

Excretory system

The excretory system is formed by the so-called pericardial glands (there are no nephridia and they lack differentiated excretory organs), with their corresponding pericardioducts (although these lack a funnel).

Nervous system

The nervous system (as in the Salvini-Plawen model) is tetraneuronic, that is, with two pedal or ventral nerve cords and two visceral or dorsal cords, both joined by transverse commissures (ladder nervous system). These cords arise from a concentration of nodes that surround the esophagus (circumenteric or periesophageal nodes). As sensory organs they have osphradia, which are organs that measure water quality and are associated with the ventral membrane that supports the gills.

Reproductive system and fertilization

They are dioecious (separate sexes) with sexual reproduction. They have a reproductive system made up of fused paired gonads (derived from the peritoneum of the anterior coelomatic sacs) with paired gonopericardial ducts associated with glands that produce the eggshell. Through pericardioducts they open into the paleal cavity. This model follows Yonge's.

Fertilization is external and they present an endolarvae (internal larvae) called pericalinas.