Class of Mollusca - Aplacophora

Worm-like, no shell, head or excretory organs mantle with chitinous cuticle or scales or spicules mantle cavity posterior. Aplacophorans are a group of small worm like molluscs devoid of any shell, known as solenogastres. They occupy ocean depths, although some shallow water species are as well as known. Their biology is poorly understood. You will very in brief study their salient features.

1. Burrowing forms feed on small organisms and deposited materials, and the creeping species feed on cnidarians. A radula may or may not be present.
2. Most forms are hermaphrodites and the gonoducts open into the mantle cavity. The eggs may develop directly into adult or pass through a trochophore larval stage.
3. They are less than 5 cm length with a poorly developed head and a cuticle- covered integument in which calcareous scales or spicules are embedded. The upward rolling of mantle margins provide worm like appearance to creeping species. The foot is much reduced in burrowing forms. The posterior end of the body has a chamber, the mantle cavity, into which anus opens. In some forms the gills are located inside the mantle cavity. There is no shell.

So far you have studied three classes of primitive molluscs - Monoplacophora, organisms with a single shell plate; Polyplacophora, organisms with many shell plates and Aplacophora, organisms with no shell plate.