162 



MOLLUSCA 



would certainly be an unexpected occurrence, to find a mar- 

 ine gasteropodous mollusca obliged to come to the surface, 

 at intervals, to respire. It will probably be found that it is 

 truly branchiferous. 



CLASS II! BRANCHIFERA. 



The molluscous animals of this class are more numerous 

 than those of the preceding. They chiefly inhabit the wa- 

 ters of the ocean, a few genera only being met with in fresh 

 water lakes and rivers. The branchiae which constitute their 

 aerating organs, exhibit numerous varieties of form, posi- 

 tion, and protection, and furnish valuable characters for their 

 methodical distribution. 



ORDER I BRANCHIAE EXTERNAL,. 



The branchiae are pedunculated, and more or less plu- 

 mose. They are moveable at the will of the animal, and, 

 in general, are capable of great alteration of form. 



GENUS DORIS. Oral tentacula two ; vent without scales. 

 The cloak is covered with retractile papillae, and separated 

 from the foot by a distinct duplicature. Towards its ante- 

 rior margin are placed the two superior tentacula. These 

 are retractile, surrounded at the base with a short sheath, 

 and supported on a slender stem, having an enlarged com- 

 pound plicated summit. The neck is short, and above the 

 mouth there is a small projecting membrane, connected at 

 each side with the oral tentacula, which are in general mi- 

 nute, and of difficult detection. The mouth is in the form 

 of a short trunk, leading to 'fleshy lips, within which the 

 tongue is placed. This last organ is covered with minute 



