The tissues and shell are transparent, permitting observation of the internal organs. In the Pterotrachaeidae the foot takes the form of a fan-shaped disc, usually furnished with a sucker. The body is compressed at the posterior end, often with a ventral “fin.” In Atlanta the foot consists of three very distinct parts: a propodium, a mesopodium, on which is a small sucker, and a metapodium, which carries the operculum. The branchiae are carried on the visceral sac, and are free in Pterotrachaea, slightly protected by the shell in Carinaria, and entirely covered in Atlanta; absent altogether in Firoloida.
The head carries two tentacles (except in Pterotrachaea), with large, highly organised eyes on short lobes at their outer base. The alimentary tract consists of a long protrusible proboscis, with a taenioglossate radula (Fig. [132], p. 227), a long oesophagus, and a slightly flexured intestine. In Atlanta the visceral sac is spiral and protected by a spiral planorbiform shell; in Carinaria the visceral sac is small, conical, protected by a very thin capuliform shell. There is no shell in Pterotrachaea or Firoloida.
The Heteropoda are dioecious. In the male there is a flagellum behind the penis, which is near the middle of the right side. Pterotrachaea lays long chains of granular eggs, and has been noticed to produce a metre’s length in a day. The eggs of Atlanta are isolated. The embryo has a deeply bilobed velum.
Fam. 1. Pterotrachaeidae.—Body long, with a caudal “fin;” branchiae dorsal, free or partly protected by a shell; foot consisting of a muscular disc, with or without a sucker.
Pterotrachaea proper has no mantle, shell, or tentacles. The branchiae are disposed round the visceral sac, at the upper part of which is the anus. In Firoloida the body is abruptly truncated behind, with a long filiform segmented caudal appendage; visceral sac at the posterior end: fin-sucker present or absent in both male and female. Cardiapoda resembles Carinaria, but the visceral sac is more posterior and is only slightly protected by a very small spiral shell. Carinaria (Fig. [279]) has a rugose translucent skin, visceral sac sub-median, apparently pedunculated, covered by a capuliform shell. The larval shell, which persists in the adult, is helicoid.
Fam. 2. Atlantidae.—Shell spiral, operculate, covering the animal. Branchiae in a dorsal cavity of the mantle; foot trilobed, with a small sucker on the mesopodium.
The shell of Atlanta is discoidal and sharply keeled, while that of Oxygyrus is nautiloid, with the spire concealed, no keel, aperture dilated.
(c) Gymnoglossa.—Radula and jaws absent; proboscis prominent, sexes probably separate, penis present. The section is probably artificial and unnecessary, the families composing it being, in all probability, Taenioglossa which have lost their radula in consequence of changed conditions of life (pp. [79], [225]).
Fig. 279.—Carinaria mediterranea Lam., Naples: a, anus; br, branchiae; f, foot; i, intestine; m, mouth; p, penis; s, sucker; sh, shell; t, tentacles. × ½.