They feed upon certain algæ, with which the bottom of the sea is covered; but they eat, also, small marine animals, such as the naked molluscs, annelids, and crustaceans.
We are the less astonished to see the Aplysiæ so gluttonous when we learn how liberally Nature has accorded to them organs of mastication, trituration, and digestion. Their mouth is formed of thick and muscular lips; a very long œsophagus or gullet succeeds, and this œsophagus does not communicate with a single stomach, but with four—one enormous membranous crop, an exceedingly muscular gizzard, with two accessary pockets, one of which terminates in the form of a sac. The gizzard has thick walls, and is furnished on the internal wall with cartilaginous quadrangular pyramids, the summits of which intertwine. This apparatus is intended to bruise the food when it reaches the third stomach. It is also armed with little hooks, the curvature of which is directed towards the entrance of the gizzard.
The genus Bulla differs materially from the Aplysiæ. They have a well-developed shell, the form of which is elegant; they are delicate in structure; their brilliant colours, consisting of red, black, or white bands, separated by many varied tints, cause these little molluscs to be much sought after for ornamental collections. The shell itself is oval or globulous, rolled up in a scroll, smooth, spotted, very thin and fragile, with a concave spiral, umbilicate, open in all its length, with a straight, wide, and cutting edge.
Figs. 183 and 184. Bulla ampulla (Linnæus).
Obtuse at its two extremities, neither the head of the animal nor the tentacles are very apparent. The gills are placed under the back, a little to the right and behind; its stomach, which alone fills a great part of the cavity of the body, presents the peculiarity, already noted in the Aplysia, of being furnished with bony pieces, evidently intended to grind the food.
Fig. 185. Bulla oblonga (Adams).
Fig. 186. Bulla aspersa (Adams).
Fig. 187. Bulla nebulosa(Gould).