376 



ORDER TECTIBRANCHIATA. APLYSIA, BULLA. 



shore, or on floating sea-weeds. A very characteristic example 

 of the group is the Aplysia, commonly termed Sea-Hare, which 



Fio. 572. APLYSIA. 



is abundant on many parts of our own coasts. Its common 

 name is probably derived from the peculiar form of the superior 

 pair of tentacula, which are flattened and hollowed like the ears 

 of a quadruped. The head has a very distinct neck. The gills 

 consist of leaflets arranged in a complex form, and situated on 

 the back beneath a fold of the mantle, which also incloses a flat 

 horny shell (Fig. 572). The digestive apparatus is very com- 

 plicated ; consisting of a membranous crop like that of Birds, a 

 gizzard having cartilaginous walls, and a third stomach beset 

 with sharp hooks in its interior. These animals feed on sea- 

 weed. They are very sluggish in their movements, but have a 

 peculiar means of defence, consisting of a deep purple liquid 

 (said by some to have acrid properties), which they can dis- 

 charge from the edge of the mantle when alarmed, and by which 

 the surrounding water is discoloured, so that the animal cannot 

 be discerned. Nearly allied to the Aplysia are the Bulla and 



FIG. 573. BULLJEA APERTA. FIG. 574. BULLA LIGNARIA (a) ; . BULLA AMPULLA (&). 



Bullcea ; these have a small calcareous shell, in which the spiral 



