GASTROPODA. 37 



cavity there is a ridge, partly muscular and partly cartilaginous, 

 which, from its resemblance to the tongue of the Yertebrata. has 

 received the same name (fig. 513). The surface of this tongue is 

 covered by a tough membrane, known as the lingual ribbon or ,-<i<hil<i, 

 on which are arranged transverse rows of plates, teeth, and hooks 

 of a characteristic form. Behind, the radula passes into a cylindrical 

 pocket, the so-called radula sheath (fig. 513 Z), which projects in a 

 tubular manner from the lower (ventral and posterior) end of the 

 buccal mass. The radula is secreted in the radula sheath. The 

 size, number, and form of the plates and teeth on the surface of the 

 radula vary in different forms, and afford important systematic 

 characters for genera and families. 



In the transverse rows of plates the so-called segments of the 

 radula membrane median, intermediate, and lateral plates may be 



FIG. 514 a, A segment of the radula of Pterofrackea Lesueurii (after Macdonald). I, ditto 



ofNeretinaJluviatilig(ateT S. Lovto). 



distinguished (fig. 514 a, b). Troschel believed that natural divi- 

 sions could be formed according to the special structure of the 

 armature of the radula. But this one-sided systematic treatment 

 requires many corrections, as has been especially shown in the case 

 of the Tsenioglossa and Rhipidoglossa. 



The vascular system presents numerous and essential variations. 

 The heart is enclosed in a special pericardium, and is usually placed on 

 one side of the middle line near the respiratory organs (fig. 515). It 

 usually consists of a conical ventricle, which gives off the aorta, and 

 of an auricle which is turned towards the respiratory organs, and into 

 which the blood passes by veins. In some Gastropods (Gastropods 

 with two gills, Haliotis, Turbo, Nerita, Fissurella, etc.), the heart 

 resembles that of the Lamellibranchs, in that there are two auricles 

 and the ventricle is pierced by the rectum. The aorta usually 

 divides into two arteries, of which one passes forward and gives off 



