296 



Mollusca. 



The central nervous system consists of paired cerebral, 

 pedal, and pleural ganglia, and a varying number of visceral ganglia, 

 which are connected with one another as shown on p. 289. In the 



opisthobranch and pulmonate 

 B Gastropods, the visceral cord 



-fi runs posteriorly, forming a 



loop between the two pleural 

 ganglia and lying below the 

 digestive tract throughout 

 the whole of its course. In 

 all the Prosobranchiata on 

 the other hand, the visceral 

 loop twists round the digestive 

 tube in a peculiar way ; the 

 nerve cord arising from the 

 left pleural ganglion runs 

 below the gut, then crosses 

 above the gut to the left side, 

 and then again crosses this 

 time above the gut to run 

 anteriorly, ending in the right 

 pleural ganglion (Fig. 243 B). 

 This peculiar arrangement of 

 the nervous system pre- 

 supposes changes in the 

 position of most of the organs. Of sense-organs, the eyes 

 have already been mentioned ; for the auditory organs the account 

 given on p. 290 for the Mollusca in general may be referred to. In 

 most of the Gastropods there is, within the mantle-cavity, a 

 specialised portion of skin covered with a peculiar epithelium, 

 often folded and richly supplied with nerves; it lies near the gill, 

 and when two gills are present two such osphradia also occur. 

 There is no doubt that they are sense organs, and they are considered 

 to be olfactory. 



With regard to the alimentary canal, it is important to 

 notice that the anus usually lies on the right side* of the mantle- 

 cavity quite asymmetrically ; only in a few isolated forms among 

 those Gastropods in which the shell is lost, is it symmetrical in 

 position {see the Opisthobranchs) . In the mouth there is a radula 



Fig. 243. Diagram of the central nervous 

 system in relation to the alimentary canal 

 {A in an Opisthobranch, Bina Proso- 

 b ranch). h cerebral, p pleural, / pedal, i 

 visceral-ganglia; t alimentary canal. — Orig. 



* Just as in other animals which are asymmetrical in some respects {e.g. Mammalia), 

 so in Gastropoda, such a departure from the usual arrangement of organs occurs, that 

 all which are usually right come to lie on the left side, and conversely (inversio 

 viscerum) ; the anus is left instead of right and so on. This happens sometimes 

 in the case of forms with shells in a left-handed spiral ; but there are forms with such 

 shells in which the torsion has not occurred, even the anus being on the right. 



