Solenogastres
From Palaeos
The Solenogastres or Neomeniomorpha are mostly small (less than 5 cm long] worm-like mollusks that live symbiotically with (or feed upon) cnidarians. As with the Caudofoveata, many of the typical molluscan characters are absent. They have no shell, eyes, or tentacles. The mantle cavity is rudimentary, and instead of the standard molluscan flattened foot there is a pedal groove, which the animal uses to creep along the bottom. They are hermaphrodites and lack ctenidia (gills) in the mantle cavity. As with the Caudofoveata, the integument contains layers of embedded calcareous spicules, possibly a link to early Cambrian coelomates. The worm-shaped body is derived from the inrolling of the mantle margins.
There are about 250 described species, probably with many more awaiting discovery. They are found primarily below 200 meters and are sometimes quite abundant in deep-sea epifauna or infauna. Many reside and feed upon cnidarians (hydroids and corals).
Despite being an important part of the deep-sea benthos, neomeniomorphs are poorly known, mainly because the difficulty of studying creatures that live in very deep water.
The Solenogastres have in the past been combined with the Caudofoveata to form the class Aplacophora. In some classifications the terms Aplacophora and Solenogastres seem to be synonymous. However the tendency now is to see them as distinct classes of mollusks.
Evolutionary Relationships
Whilst adult physiology may differ greatly between different groups of organisms, embryonic ontogeny may show evolutionary relationships, a fact discovered by the Darwinian philosopher Ernst Haeckel in the 19th century. Here larval ontology indicates the relation between Solengastres and Polyplacophora on the one hand, and sclerite-bearing "molluscomorphs" on the other. Salvini-Plawen (1980) provides an illustration (below) showing the similarity between larval forms of each class.
Differentiation of the mantle cover in just metamorphosed Nematomenia banyulensis (Solengastres - A and B) and Middendorffia caprearum (Polyplacophora - C and D). Pl shell plates in formation through the coalescence of the scaly bodies Sp arranged in seven transverse rows. Caption and figure from Salvini-Plawen 1980 p.252
Similarly, a recent discovery of a neomenioid postlarva (Scheltema & Ivanov, 2002) shows it has six iterated, transverse groups of spicules and seven regions devoid of spicules. These resemble the shell fields in developing polyplacophorans, and spicule arrangement is compared to sclerite arrangement on the Cambrian fossils Wiwaxia corrugata and Halkieria evangelista and to the spines and shell plates of the Silurian Acaenoplax hayae. Such iterative morphogenesis was probably a common theme in late Ediacaran and Cambrian procoelomate/molluscomorph animals and shows how these groups are related.
It is still unclear to what extent the Solenogastres and Caudofoveata are specialized and to what extent they are primitive, even "living fossils". Both these groups have been hypothesized to be ancient, pre-Tethyan deep-sea forms, perhaps relics of the original Cambrian procoelomate radiation. Dzik (1993), in contrast, suggests they developed from aberrant chitons like the Silurian Carnicoelus gazdzickii (id. at p.368, fig.12), but there is no evidence a shell was ever present. Barnes (1980). Salvini-Plawen (1980) sees Solenogastres as the sister group to all shelled mollusks (Testaria), while a cladistic study by Haszprunar (2000) indicates that Solenogastres are even more underived than Caudofoveata.
