HIGH SCHOOL ZOOLOGY. 



233 



to that of tlie Lamellibrancbs), which, in the simplest cases, is 

 formed of a single conical piece ; in most Gastropods, however, 

 the shell is spiral, and the visceral mass has, in the course of 

 development, become twisted in the same direction, so as to cause 

 the intestine e. g. to open anterioi-ly. This twist results in the 

 suppression of the gill and nephridium of one side, and also in 





Fig. 15S. — Diagram of Limnaca, to show the course of the intestine and the 

 arrangement of the nerve-ganglia in the head. 



r, floor of the mouth occupied by the radula ; r', a row of lingual teeth ; ff, posi- 

 tion of apertures of reproductive organs ; m, the free mantle margin ; 7«i the line of 

 fusion of the mantle with the body-wall bounding the lung in front ; I, the aperture of 

 the lung : a, the anus ; S, the stomach, receiving tne tubes of the liver which occupies 

 the apex of the shell : C, the cerebral, pi, the pleural, v, the visceral ganglion ; be 

 tween the two pleural are the two pedal, and between the visceral, the single abdom- 

 inal ganglion. 



the asymmetrical situation of the heart. The direction of the 

 twist determines which side of the body shall be afiected ; it is 

 sometimes towards the right (dexiotropous), but generally 

 towards the left (leiotropous), in which case the organs of the 

 right side are retained at the expense of those of the left. 



The mantle-cavity, which is so roomy in the Acephala, is 

 much restricted in the Gastropods, and is confined to the sides 

 of the visceral mass. The mantle-margin is rarely free, but 

 generally forms an enclosed space opening externally by an 

 aperture or tube, which lies on the right side in left- twisted 

 (leiotropous) forms. 



Apart from the asymmetry referred to, the most striking dlf- 

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