418 MOLLUSCA: GASTEROPODA. 



body, are wholly uncovered. These firo tlie Doridse 

 (Doris, etc., Fig. 612), the Tritonidre (Tritonin, etc., Fig. 

 613), the yEolidre (dZolis, etc., Fig. 611), the Elysiadne 

 (Elysia, etc.), etc. 



4. The HETEROPODA or Nucleobranchiata include the 

 Firolidre (Firola, Carinaria, etc.), and the Atlantidre 

 (Atlanta, etc., Fig. 608). These, as before indicated 

 on p. 407, are marine gasteropods. which have the 

 "foot"' so modified as to be a fin-like tail, or fan-shaped 

 ventral fin, by means of which they swim at the surface 

 of the sea. They are called Nueleobranchs because their 

 respiratory organs are in the form of a sort of nucleus. 



5. The PTEROPODA by some regarded as a Class, and 

 by others as a Sub-class have their locomotive organ so 

 modified as to appear like wings. These curious mollusks 

 live in the open sea, moving in immense swarms. They 

 include the Hyaleidaa (Ilyalea, etc., Fig. 614), the Lima- 

 cinidiTe (Limacina, etc., Fig. 615), and the Cliidse (Clio, 

 etc., Fig. 616). 



The two first-named families Hyaleidae and Limacini- 

 dse are the Thecosomata of authors, so called because 

 some of the genera have a shell from the Gr. theke, a 

 case, and soma, body. And the last-named family Cliida3 

 is the group called Gymnosomata, because the animal 

 is naked, or without a shell from the Gr. yumnoSj naked, 

 etc. 



