XTTl] CAUSES OF ASYMMETRY 297 



allies of the snail the Whelk, Limpet and others forming the group 

 Prosobranehiata the inequality of the two sides of the visceral 

 hump is carried to such an extent that the anus is brought right 

 round so as to open nearly over the middle of the neck ; and where, 

 as in the Ear-shell (Haliotis), there are two gills, the left becomes 

 pushed over to the right side and the gill belonging to the right 

 side becomes displaced to the. left and both gills come to be in 

 front of the heart. Since the visceral ganglia are connected with 

 the bases of the gills, one side of the visceral loop becomes pulled 

 over the other in consequence of the displacement of the gills 

 (Fig. 135). This condition of the nervous system is called the 

 streptoneurous condition ; it exists in one division of the 

 Molluscs which are ordinarily termed "sea-snails." Such Mollusca 

 with a twisted loop are termed Prosobranehiata (Gr. Trpw'o-ov., 

 in front), and though most of these have only one gill, the twisting 

 of the visceral loop may be regarded as a proof that they originally 

 had two. In the other large division of the sea-snails, the Opistho- 

 branchiata (Gr. o7rio-0ev, behind), the shell is generally small or has 

 quite disappeared, and since where this has taken place there seems 

 to have been a tendency to undo the twisting, the anus becomes 

 pushed back to nearly the posterior end of the body and the visceral 

 loop becomes straightened out and shortened. The gill when present 

 is behind the heart. There is reason to believe that this last 

 process has gone on in the snail, though it has kept its shell. It 

 appears then that the curious . spiral form of part of the body and 

 the inequality of the sides have something to do with the posses- 

 sion of a large shell by a crawling animal. We do not understand 

 very clearly how the one thing has brought about the other, but 

 we can understand that there would be a tendency in a tall visceral 

 hump to topple over to the one side or the other and thus exercise 

 a greater strain on one side than on the other and the side which 

 was stretched would tend to grow faster than the other. If we 

 start therefore with a simple cap-like shell with a round opening 

 and if this shell grows as we know it does by the prolongation of 

 the lips of the opening and if the lip grows faster on one side than 

 on the other a spiral shell must result. Certain it is, at any rate, 

 that the only existing Mollusca which possess large coiled shells 

 and yet are bilaterally symmetrical, are the pearly Nautilus and 

 another rare Cuttle-fish (Spirula), which do not crawl but swim. 



The class or primary division of the Mollusca to which the snail 

 belongs is called the Gastropoda, on account of the flat smooth 



