. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. 310 MANUAL OF ZOOLOGY. hinder end of the strap is ;—(Huxley.) The teeth of the odontophore ("lingual teeth") are composed of silica, and are usually arranged in a central (" rachidian") and two lateral ("pleural") rows. The mouth leads by a gullet into a distinct stomach, which is sometimes provided with calcareous plates for the trituration of the food. The intestine is long, and its first flexure is commonly " haemal," or towa
. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. 310 MANUAL OF ZOOLOGY. hinder end of the strap is ;—(Huxley.) The teeth of the odontophore ("lingual teeth") are composed of silica, and are usually arranged in a central (" rachidian") and two lateral ("pleural") rows. The mouth leads by a gullet into a distinct stomach, which is sometimes provided with calcareous plates for the trituration of the food. The intestine is long, and its first flexure is commonly " haemal," or towards that side of the body on which the heart is situated; though in some the flexure is " ; Distinct salivary glands are usually present, and the liver is well developed. A distinct heart is usually present, composed of an auricle and ventricle. In many Gasteropods it has been shown that the blood-vessels form closed tubes, and that the arteries and veins are connected by an intermediate system of capillaries, instead of merely communicating through the interstices and lacunje between the tissues. It seems also certain that, in general at any rate, there is no direct connection between the blood-vessels and the outer medium, though, in some cases, such a communication seems undoubtedly to exist. Respira- tion is very variously effected; one great division (Branchio- gasterofoda) being constructed to breathe air by means of water; whilst in another section (Pulmogasteropodd) the respi- ration is aerial. In the former division respiration may be effected in three ways. Firstly, there may be no specialised respiratory organ, the blood being simply exposed to the water. Fig. log.—AmpitUaria catial'iailain, one of the Apple-shells, o Operculum; s Respiratory siphon. in the thin walls of the mantle-cavity (as in some of the Hetero- poda). Secondly, the respiratory organs may be in the form of outwai:d processes of the integument, exposed in tufts on the back and
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