ALLIES OF THE PELECYPODA : MOLLUSCA 187 



shell in Dendronotus, but the presence of other organs, 

 including the rasping-tongue, proves their relationship to the 

 garden-slug. 



Nudibranchs deposit their eggs in protected places, fasten- 

 ing them with mucus against rocks and seaweeds. One may 

 recognize the egg-mass of Dendronotus by its salmon-pink 

 color and its coiled arrangement. 



The color of this animal is rich brown ; the same colors 

 occur in its immediate environment. Quite as valuable as 

 the color-resemblance, in protecting the nudibranch against 

 ravenous fish, is the marked similarity of the cerata, in form, 

 to the delicate branches of certain seaweeds. The slowness of 

 the animal's movements must also aid it in escaping notice. 



Definition of Gasteropoda (Gr. gaster, stomach ; pous 

 (2>od), foot). The class represented by the snails, the slug, 

 and the nudibranch is given the name Gr aster op' o da. The 

 class-name is only figuratively correct. The ventral surface, 

 not the stomach, is modified to form a locomotor organ, 

 the foot. 



An important characteristic of the Gasteropoda is the 

 unsymmetrical arrangement of organs. With the exception 

 of the mouth and the opening of the mucus-gland, all the 

 openings of the body are on the right side, even in cases like 

 the slug and nudibranch, where the general form of the body 

 is bilaterally symmetrical. The asymmetry (lack of symme- 

 try) of the organs is directly connected with the existence 

 (present or past) of a shell. When a shell occurs it is com- 

 posed of one piece, and the characteristic form is spiral. A 

 shell-forming organ, the mantle, is usually present. 



The body of Gasteropoda is not divided into somites, but 

 the head is slightly marked off from the rest of the animal, 

 and is provided with eyes and unsegmented tentacles. A 

 tongue-like organ bears a ribbon of minute teeth for tear- 

 ing food. 



