MOLLUSCA : GASTEROPODA. 



407 



there is an elongated retractile proboscis, with ear-sacs, contain- 

 ing otoliths, at its base. The mouth is sometimes furnished 

 with horny jaws, and is (with extremely few exceptions) pro- 

 vided with a singular masticatory apparatus, which is variously 

 known as the "lingual ribbon," the "tongue," the "odonto- 

 phore," or the "radula." This consists of a longer or shorter 

 ribbon - shaped structure, which is attached behind to the 

 bottom of a secreting sac or sheath, situated on the lower 

 wall of the pharynx posteriorly. The lingual ribbon extends 

 forwards along the inferior wall of the pharynx, being supported 

 by a species of cartilaginous cushion, over which it can be 

 made to work backwards and forwards by appropriate muscles. 

 It carries a great number of hook-shaped teeth arranged in 

 transverse rows, there generally being a principal central and 

 two or more lateral rows (fig. 213). These teeth formerly 

 supposed to be siliceous, are now known to 

 be mainly chitinous, and their form and dis- 

 position are so various and so constant in 

 different forms, that they afford very valuable 

 help in classification. The mouth leads by a 

 gullet into a distinct stomach, which is some- 

 times provided with cartilaginous or calcare- 

 ous plates for the trituration of the food. The 

 intestine is long, and its first flexure is com- 

 monly " 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 Fig. 213. Fragment 

 liver is well developed. ' 



A distinct heart is almost always present, 



, - . , , J . r T ' 



composed of an auricle and ventricle. In 

 many Gasteropods it has been shown that the J^d. 

 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 inter- 

 stices and lacunae 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- 

 gasteropodd) being constructed to breathe air by means of water; 

 whilst in another section {Pulmogasteropoda) the respiration 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 in the thin 



of r , the common 



Whelk (Buccin-um 



undatnm), magni- 

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