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.
In the Cephalopoda, from κεφαλὴ, head, and ποῦϛ, foot, the locomotive apparatus consists of arms, or tentacles, which surround the mouth in numbers more or less considerable. The Cuttle-fish (Sepia), and the Poulpes (Octopoda) are types of this last class.
The Molluscous Gasteropoda have the organs of respiration formed for aerial respiration, or for respiration under water.
This physiological arrangement involves important differences in internal organization in these molluscs, and renders it convenient to divide them into two secondary groups; namely, Pulmonary Gasteropods, which breathe in the air, and by a species of lung, and Non-pulmonary Gasteropods, which breathe in the water, by means of branchiæ or gills.