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. [Illustration: 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. [Illustration: Fig. 185. Bulla oblonga (Adams).] [Illustration: Fig. 186. Bulla aspersa (Adams).] [Illustration: Fig. 187. Bulla nebulosa (Gould).] The Bullæ can swim with facility in deep water, but they evidently prefer the shallows and a sandy bottom, feeding upon smaller molluscs. They are found in every sea, but they abound chiefly in the Indian Ocean and Oceania. Some species, however, such as _Bulla ampulla_ (Figs. 183 and 184), the shell of which is shaded grey and brown, and the Water-drop (_Bulla hydratis_), inhabit European seas. _Bulla oblonga_ and _Bulla aspersa_ (Adams), and _Bulla nebulosa_ (Gould), represented in Figs. 185, 186, and 187, are also well-known species. We take leave of our little friends the Headless Mollusca or _Acephalæ_, and direct our attention to those molluscs to which Nature has been more generous, and furnished with a head. This head, however, is still carried humbly; it is not yet _os sublime dedit_; it is drawn along an inch or so from the ground, and in no respect resembles the proud and magnificent organ which crowns and adorns the body of the greater and more perfectly organized animals. The organization of the Cephalous Mollusca present three principal types, which has led to their being divided into three classes, after their more salient characteristics of form and locomotive apparatus; namely, _Gasteropoda_, _Pteropoda_, and _Cephalopoda_. In the class _Gasteropoda_ (from γαστὴρ, _belly_, ποῦϛ, gen.ποδὸϛ, _foot_) the locomotive apparatus consists of a flattened muscular disk, placed under the belly of the animal, aided by which it creeps. The Snail (_Helix_), the Slug (_Limax_), and the Cowrie, (_Cyprea_), are types of this class. In the _Pteropoda_, from πτερὸν, _wing_, and ποῦϛ, _foot_, the locomotive apparatus assumes the form of wings, or membranous swimming-fins, placed on each side of the neck. The _Hyalea_ and _Clio_ are types of this class.