264 MANUAL OF ZOOLOGY. 



lidce (Bubble-shells), Aplysiadce (Sea-hares), Pleurobrancliidce 

 and PJiyllidiadce. 



In the second section, that of the Nudibranchiata, the animal 

 is destitute of a shell, except in the embryo condition, and the 

 branchiae are always placed externally on the back or sides of 

 the body. This section comprises the families, Doridce (Sea- 

 lemons), Tritoniadce, ^Eolidce, Phyllirhoidce, and Ulysiadce. 



ORDER III. NUCLEOBRANCHIATA OR HETEROPODA. This order 

 is denned by the following characteristics : Animal pro- 

 vided with a shell, or not, free-swimming and pelagic ; lo- 

 comotion effected by a fin-like tail, or by a fan-shaped, verti- 

 cally-flattened ventral fin. 



Fig. 80. Heteropoda. Carinarin rymbium. p. Proboscis; (. Tentacles; 

 6. Branchiae ; s. Shell ; /. Foot ; d. Disc. (After Woodward.) 



The Heteropoda are pelagic in their habits, and are found 

 swimming at the surface of the sea. They are to be regarded 

 as the most highly organised of all the Gasteropoda, at the 

 same time that they are not the most typical members of the 

 class. Some of them can retire completely within their shells, 

 closing them with an operculum, but most have large bodies, 

 and the shell is either small or entirely wanting. They swim 

 by means of a flattened ventral fin, or by an elongated tail, and 

 adhere at pleasure to sea- weed by a small sucker situated on 

 the side of the fin. These organs are merely modifications of 

 the foot of the ordinary Gasteropods ; the fin-like tail being the 

 'metapodium' (as shown by its occasionally carrying an oper- 

 culum), the sucker being the 'mesopodium,' and the ventral fin 

 being a modified ' propodium.' Respiration is sometimes carried 

 on by distinct branchiae, but in many cases these are wanting, 

 and the function is performed simply by the walls of the pallial 

 chamber. 



The Heteropoda are divided into the two families Firolidce 



