HETEROPODA 42 I 



therefore probably derived from the Prosobranchiata, but they 

 are highly specialised forms. Pelseneer considers them far more 

 widely removed from the Streptoneura than the Pteropoda are 

 from the Euthyneura. They swim on the surface " upside down," 

 i.e. with the ventral side uppermost. 



The tissues and shell are transparent, permitting observation 

 of the internal organs. In the Pterotracliaeidae 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 

 metapodiuni, which carries the operculum. The branchiae are 

 carried on the visceral sac, and are free in Fterotrachaea, slightly 

 protected by the shell in Carinaria, and entirely covered in 

 Atlanta ; absent altogether in Firoloida. 



The head carries two tentacles (except in Fterotrachaea^, 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 Carin- 

 aria the visceral sac is small, conical, protected by a very thin 

 capuliform shell. There is no shell in Fterotrachaea 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. Fterotrachaea 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. Fterotrachaeidac. — Body long, with a caudal " fin ; " 

 branchiae dorsal, free or partly protected by a shell ; foot consist- 

 ing of a muscular disc, with or without a sucker. 



Fterotrachaea 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 trun- 

 cated 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 



