The most unique feature is the proboscis, which normally lies in a special cavity until pushed out by hydrostatic pressure to stab some prey. They are microscopic, multicelled aquatic animals that superficially resemble protozoans. Structurally they are remarkably complex, possessing a crown of cilia, jaws, digestive tract, protonephridia, nerve cells, eyespots, and muscles. The coelom forms from tissue at the sides of the gut near the blastopore. The coelom forms from outpouchings of the gut wall.
Mollusks are the second-largest invertebrate group.
Most of the other organs are located in the spiraled or cone-shaped shell. As the larvae develop, the internal organs undergo a unique twisting, or torsion, that places the anus anteriorly near the mouth. Nudibranchs seem to have undergone detorsion and have exposed ctenidia. Chitons Chitons are marine mollusks with an oval body covered dorsally with eight movable plates and ventrally by a large adhesive foot. The radula is used to scrape small animals from the rocks along the seashore.