480 MOLLUSCA 



which contains a raspinj;' organ called the radula: an ojsophagus, a 

 stomach with a large liver, and an intestine form the remainder of the 

 alimentary canal. The radula (Fig. 742,2) is a chitinous ribbon set 

 with minute calcareous teeth in regular longitudinal and transverse 

 rows which works over a muscular tongue. The teeth vary in number 

 from one in Clicetodcrma to thousands in many snails and are often of 

 great importance in classification, inasmuch as in each species they 

 have usually a characteristic arrangement and shape. In each trans- 

 verse row there is typically a single central tooth, on each side of 

 which are one or more laterals, and at the side of these, one or more 

 marginals (Fig. 830). The respiratory system consists typically of a 

 pair of ctenidia, or comb-shaped gills in the mantle cavity (Fig. 821, A). 

 They are modified in various ways among the different mollusks and in 

 pulmonate snails, nudibranchs, and some others, are wanting, respira- 

 tion being performed by lungs or by other organs. The circulatory 

 system consists of a heart, arteries which distribute the blood to lacunar 

 spaces in the tissues, and veins which take it from these spaces to the 

 kidneys and respiratory organs and thence to the heart. The heart con- 

 sists typically of a muscular ventricle (Fig. 821,3), which drives the 

 blood through the arteries (4), and one or two pair of auricles (2), which 

 receive it from the gills. Snails with but one gill or lung have also but 

 one auricle. The heart thus always contains arterial blood. The ex- 

 cretory system consists typically of paired sac-like kidneys (nephridia) 

 which open into the mantle cavity at their hinder ends (Fig. 746, 14) 

 and into the pericardial space (the coelom) at the forward. The nervous 

 system contains four principal pairs of ganglia with nerves joining 

 them. These are the cerebral ganglia (Fig. 740, 10), just above the 

 oesophagus, which form the brain, the pedal (7), the visceral (6), and 

 the pleural ganglia. The cerebral ganglia are joined with the pedal and 

 visceral ganglia by nerve connectives; in gastropods the pleural ganglia 

 lie in the course of the cerebro-visceral connectives; in pelecypods they 

 are joined with the cerebral and in cephalopods with the visceral 

 ganglia. 



The most important special sense organs consist of camera-obscura 

 eyes and tentacles usually on the head, but in the pelecypods in the edge 

 of the mantle when present, lithoeysts in the foot, and osphradia, organs 

 of taste or smell, near the base of the gills. 



The gonad is single in all mollusks except the pelecypods and 

 Solenogastres, in which it is paired; the ducts leading from the gonad 

 open usually into the mantle cavity, and are either paired or single. 

 The sexes are usually separate, although numerous hermaphrodites 

 (pulmonates, opisthobranchs) occur. The young animal in most 



