302 



LOWER INVERTEBRATES. 



tlie outside of the tentacles near their junction with the head. The shells are mostly- 

 small, and possess but little interest ; a large proportion are fossil, ranging from the 

 carboniferous to the present time. 



The BuLLiD^ embraces much larger forms, in which the ventricose 

 shell is coiled in a spiral in which the sph-e is internal. The shell, in 

 many forms, is spirally banded or spotted, and is more or less concealed 

 by the mantle and epipodia. Th-e lingual ribbon bears one median and 

 many lateral teeth. In BuUa the eyes are sessile on the middle of the 

 frontal fold formed by the united bases of the tentacles. The species 

 ^^?e\i^kammea^ frequent sandy and muddy bottoms near the shore, even going into 

 brackish water. At the retreat of the tide they burrow into the mud or 

 hide themselves beneath masses of seaweed. On our east coast is found _B. solitaria, 

 a brownish spotted form. Cylichna, which possibly deserves family rank, is repre- 

 sented on our shores by several small cylindrical shells which frequent slightly deeper 

 water than the Bullas. They move very slowly. Saminea may be readily separated 

 from Bulla by the lack of color in the shell. 





Fig. 344. — Accra buUcUa. 



In the Philinid^ the bases of the tentacles are united to form a broad cephalic 

 disc. The shell, which is covered by the mantle and epipodia, is shaped like that of 

 Bulla, but scarcely forms a single whorl ; in some it is internal and in others external. 

 Eyes may be present or absent. The species are found in water of moderate depth, 

 many species of Philine frequenting the shallow water along the shores. 



The Aplysiid^ embraces slug-like forms known in popular parlance as 'sea- 

 hares.' The shell is small or wanting, and when present is covered by the mantle. 

 The stomach is armed with hardened teeth which play no unimportant part in prepar- 

 ing the food for digestion. The animals feed principally on other molluscs, especially 

 on species of Acera (one of the Bullidse). Aplysia, the principal genus, has a pointed 

 oval shell, and the epipodia are extended in swimming. lu one species {A. cameltis) 

 numerous small glands are found beneath the free edge of the mantle which secrete 

 the purple for which these animals were celebrated among the ancients. Near the 

 base of the gill is the outlet of a gland, the secretion of which is said to be poisonous, 

 but whether any of the sea-hares have the toxic effects attributed to them, or even 

 have any poisonous qualities, is yet to be determined. Certain it is that all of the 

 group are not poisonous, for one species forms an article of diet among the South 

 Sea Islands. Some of the European species have a very nauseous smell. 



