MOLLUSCA : GASTEROPODA. 



331 



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 (fig. 133), 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 (fig. 133, //) is usually present, composed of 



Fig. 133. Diagrammatic section of a Whelk, a Mouth, with masticatory apparatus ; 

 b Salivary glands; c Stomach; d d Intestine, surrounded by the liver, and ter- 

 minating in the anus (e) ; g Gill ; h Heart ; ./Nervous ganglion. 



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 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 



