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Among the shell-less molluscs ("Aplacophora") there are about 340 of the most primordial molluscs to be found on earth. As their scientific name Aplacophora literally states, the members of this group actually are devoid of a shell and in that regard are unique among molluscs. Instead of a shell, their exterior is protected by a sturdy skin, the cuticula, reinforced with calcareous spines or scales, which give the creature a glossy sheen.
Two classes are grouped among the aplacophoran Molluscs: Caudofoveata and Solenogastres. All aplacophoran molluscs exclusively live in the sea, caudofoveatans living on the ocean floor feeding on micro-organisms and detritus, solenogasters living and feeding on corals and other cnidarians.
 Many solenogasters (Epimenia babai) live and feed on corals. Source: Atwiki.jp. |
Shell-less molluscs are opposed to all other molluscs which are combined as shell-bearing molluscs (Testaria). Apart from molluscs with complete shells, such as snails, clams and cuttlefish, there are also the chitons (Polyplacophora). Those do not have a complete shell, but their back is protected by eight shell plates.
The mollusc system. Source: Nordsieck, R.
(2008), see bottom of page.
All other mollusc classes possess a real one-part shell and therefore are combined as Conchifera, shell-bearing molluscs. Among those there are the snails and slugs (Gastropoda), mussels and clams (Bivalvia), scaphopods (Scaphopoda) and squids, octopuses and cuttlefish (Cephalopoda). The bivalve shell only divided itself into two halves but later in evolution. Also a class of conchiferan molluscs is the class of Neopilina and its relatives (Tryblidia or "Monoplacophora"). Neopilina is a living fossil, one of the last species of its kind, which is assumed to stand near the most primordial of conchiferan molluscs, though in the Tryblidia class as well there are many derived characters.
In further theories towards a systematics of molluscs it has been examined, if there should be a combined group of spiny molluscs ("Aculifera"), made from shell-less molluscs ("Aplacophora") and chitons (Polyplacophora) (neither of both has a complete one-part shell and retains the sturdy cuticula), as opposed to the "real" conchiferan molluscs.
 Paraphyletic group. |
It is assumed, though, that the group of Aculifera would be as paraphyletic as the Aplacophora. That means that in Aculifera, as well as in Aplacophora, there are not all descendants of a common ancestor. As phylogenetic analysis attempts to assemble complete groups of descendants to one preferably specified common ancestor, paraphyletic groups usually are not accepted, as opposed to monophyletic groups which do contain all descendants of such an ancestor.
It has proven very difficult to put the shell-less molluscs in order, as there naturally are very little fossil remnants to work with. All other mollusc groups have been proven since the Cambrian period more than 540 Mio. years ago, so the assumption appears to be allowed that the shell-less molluscs were most likely present at that time as well.
 Spathoderma californicum, a caudofoveatan. Source: MBARI. |
Finally it shall be remarked that of course not all molluscs without a shell automatically are placed among the shell-less molluscs! In numerous groups (especially gastropods and cephalopods) there have been several occasions during evolution, when the shell was reduced for the sake of higher mobility. But on one side, the shell can be seen in the embryonic stage, on the other those groups’ basic construction places them into their class anyway, with or without shell.
List of
publications).Salvini-Plawen, L.; Mizzaro-Wimmer, M.: "Praktische Malakologie - Beiträge zur vergleichend anatomischen Bearbeitung der Mollusken", Vienna 2001.