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In these animals the head is distinctly marked, being separated, by a contraction or depression behind it, from the rest of the body. The feelers, so prominent on the anterior extremity of the Gasteropoda, are suppressed in Cephalopoda, and the eyes are consequently brought immediately on the side of the head, and are very large in proportion to the size of the animal.
Not only have the invertebrata, as shown by geological data, altered at a less rapid rate than the vertebrata, but if we take one of the classes of the former, as for example the mollusca, we find those of more simple structure to have varied at a slower rate than those of a higher and more complex organisation; the Brachiopoda, for example, more slowly than the lamellibranchiate bivalves, while the latter have been more persistent than the univalves, whether gasteropoda or cephalopoda.
Jeffreys, J. Gwyn, on the form of the shell in the sexes of the Gasteropoda; on the influence of light upon the colours of shells. Jelly-fish, bright colours of some. Jenner, Dr., on the voice of the rook; on the finding of new mates by magpies; on retardation of the generative functions in birds.
He has not himself given us an analysis of the results of his treatise, but the following tables have been drawn up for me by Mr. Wood's careful examination of 442 species of mollusca has led. Brachiopoda : 6. Lamellibranchia : 206. Gasteropoda : 230. Norwich Crag : 81. Red Crag : 225. Coralline Crag : 327. Norwich Crag : 69 : 12 : 85%. Red Crag : 130 : 95 : 57%. Coralline Crag : 168 : 159 : 51%.
Among such indications I may remind you of the predominance of Holostome Gasteropoda in the older rocks as compared with that of Siphonostome Gasteropoda in the later.
The Gasteropoda, though capable of locomotion and furnished with imperfect eyes, do not appear to be endowed with sufficient mental powers for the members of the same sex to struggle together in rivalry, and thus to acquire secondary sexual characters.
Shells, difference in form of, in male and female Gasteropoda; beautiful colours and shapes of. Shield-drake, pairing with a common duck; New Zealand, sexes and young of. Shooter, J., on the Kaffirs; on the marriage-customs of the Kaffirs. Shrew-mice, odour of. Shrike, Drongo. Shrikes, characters of young. Shuckard, W.E., on sexual differences in the wings of Hymenoptera.
The second class in this type is that of Gasteropoda, so named from the fleshy muscular expansion on which they move, and which is therefore called a foot: a very inappropriate name; since it has no relation or resemblance to a foot, though it is used as a locomotive organ. This class includes all the Snails, Slugs, Cockles, Conchs, Periwinkles, Whelks, Limpets, and the like.
Many of these animals are destitute of any shell; and where they have a shell, it is not coiled from right to left or from left to right as in the spiral of the Gasteropoda, but from behind forwards as in the Nautilus.
Pteropods constituted another class in his division of the type of Mollusks; but these animals, again, form only an order in the class of Gasteropoda, as Brachiopods form an order in the class of Acephala. In the third division of the Animal Kingdom, the Articulates, we have again three classes: Worms, Crustacea, and Insects.
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