| Fig. 33.—Section of the shell of Triton, Cuv. (From Owen.) |
| a, Apex. ac, Siphonal notch of the mouth of the shell. ac to pc, Mouth of the shell. w, w, Whorls of the shell. s, s. Sutures. |
| Occupying the axis, and exposed by the section, is seen the “columella” or spiral pillar. The upper whorls of the shell are seen to be divided into separate chambers by the formation of successively formed “septa.” |
Fam. 53.—Pyramidellidae. Summit of spire heterostrophic; a projection, the mentum, between head and foot; operculum present. Pyramidella. Turbonilla. Odostomia, British. Myxa.
Fam. 54.—Eulimidae. Visceral mass still coiled spirally; shell thin and shining. Eulima, foot well developed, with an operculum, animal usually free, but some live in the digestive cavity of Holothurians. Mucronalia, foot reduced, but still operculate, eyes present, animal fixed by its very long proboscis which is deeply buried in the tissues of an Echinoderm, no pseudopallium. 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.
Fam. 55.—Entoconchidae. 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. Entocolax, mouth at free extremity, animal fixed by aboral orifice of pseudopallium, Pacific. Entoconcha, body elongated and tubular, animal fixed by the oral extremity, protandric hermaphrodite, parasitic in testes of Holothurians causing their abortion. Enteroxenos, no pseudopallium and no intestine, hermaphrodite, larvae with operculum.
Tribe 2.—Heteropoda. Pelagic Taenioglossa with foot large and laterally compressed to form a fin.
Fam. 1. Atlantidae. Visceral sac and shell coiled in one plane; foot divided transversely into two parts, posterior part bearing an operculum, anterior part forming a fin provided with a sucker. Atlanta. Oxygyrus.
Fam. 2.—Carinariidae. Visceral sac and shell small in proportion to the rest of the body, which cannot be withdrawn into the shell; foot elongated, fin-shaped, with sucker, but without operculum. Carinaria. Cardiopoda.
Fam. 3.—Pterotrachaeidae. Visceral sac very much reduced; without shell or mantle; anus posterior; foot provided with sucker in male only. Pterotrachaea. Firoloida. Pterosoma.
Sub-order 2.—Stenoglossa. Radula narrow with one lateral tooth on each side, and one median tooth or none.
Tribe 1.—Rachiglossa. Radula with a median tooth and a single tooth on each side of it. Formula 1 : 1 : 1. Rudimentary jaws present.