310 MANUAL OF ZOOLOGY. 



hinder end of the strap is lodged." — (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 " neural." 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 sides of the animal (as in the Ntidibranchiata). 

 Thirdly, the respiratory organs are in the form of pectinated 



