H, The mantle-skirt reflected over the sides branchia the mouth-bearing cylinder is in C, Head, the letter placed near the right eye.
Head very large, not marked off from the body; neither branchia nor suckers; fins situated near the middle of the body.
Stylifer, the operculum is lost, animal fixed by a large proboscis which forms a pseudopallium covering the whole shell except the extremity of the spire, parasitic on all groups of Echinoderms. Entosiphon, visceral mass still coiled; shell much reduced, proboscis very long forming a pseudopallium which covers the whole body and projects beyond in the form of a siphon, foot and nervous system present, eyes, branchia and anus absent, parasite in the Holothurian Deima blakei in the Indian Ocean.
Posterior branchia triradiate.
Anus and branchia posterior, below the mantle-border.
In these there are neither branchia nor osphradium, and the pallial chamber which retains its large open ing serves as a lung.
Br, Secondary branchia surrounding the anus.
The heart is usually posterior to the branchia (proso-branchiate).
The sub-class is now divided into two orders: the Aspidobranchia in which the branchia or ctenidium is bipectinate and attached only at its base, and the Pectinibranchia in which the ctenidium is monopectinate and attached to the mantle throughout its length.
No shell; visceral mass not coiled; no sensory organs, nervous system, branchia or anus; body reduced to a more or less tubular sac; hermaphrodite and viviparous; parasitic in Holothurians; larvae are veligers, with shell and operculum.