Advanced text-book of zoology for junior students

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1878 Excerpt: ...disc upon which they creep about, as may readily be observed in the common slugs. Some, however, have the foot much modified and adapted for swimming. In many cases, also, the foot carries behind a horny or shelly plate which is known as the "operculum" (fig. 127, o), and which serves to close the shell when the animal is withdrawn within it. The head in most of the Gasteropoda, unlike the Bivalves, is very distinctly marked out, and carries two long feelers, and two eyes, often placed upon stalks (fig. 127, A). The mouth, also, differs from that of the Bivalves in being furnished with a singular apparatus of teeth, constituting what is known as the "odontophore" (fig. 128), or "lingual ribbon." This consists essentially of a number of siliceous teeth, of different shapes in different species, supported upon a kind of strap which can be made to work backwards and forwards over a cartilaginous cushion, thus acting like a chainsaw. In addition to the odontophore there are sometimes horny jaws as well. The mouth leads by a gullet into a distinct stomach, which sometimes is provided with calcareous platesfor grinding down the food. The intestine is long, and always terminates in a distinct anal aperture. Distinct salivary glands are usually present, and the liver is well developed. A distinct heart is almost always present, and consists of two chambers, an auricle and a ventricle. Respiration is very variously effected--one great division being constructed to breathe air by means of water, whilst in another section the respiration is aerial. In the former of these--often spoken of as the "branchiate" Gasteropods--respiration may be carried on in three ways. Firstly, there may be no special breathingorgan, the blood being ...