Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 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 projec
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 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(a£teT 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 t
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