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.
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.