HETEROPODA 42 1 



therefore probably derived from the Prosobranchiata, but they 

 are highly specialised forms. Pelseiieer considers them far more 

 widel}^ 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 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 pro- 

 boscis, 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 Ptero- 

 trachaea 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 consist- 

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



Pterotrachaea i^roipev 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 



