HICKSON : TORSION IN MOU.USCA, 



which the nervous system had become involved in the twisting of 

 the body. This would be represented by the expression 



STREPTONEURA EUTHYNEURA 



ANCESTRAL ISOPLEURAN GASTROPOD. 



The more recent investigations of some of the Euthyneura have 

 thrown grave doubts upon the correctness of this opinion, and make 

 it almost certain that the Euthyneura must have passed through a 

 primitive streptoneurous condition. 



The line of investigation has been somewhat as follows. The 

 order Opisthobranchiata includes those Euthyneura in which the heart 

 usually lies in front of the gill, instead of behind, or at the side of it, 

 as it does in the Streptoneura. Some of these, such as Action. Bulla, 

 Scaphander, etc., have a shell that is spirally coiled. Now, if the first 

 theory enunciated above were true, the spiral shell of the Opistho- 

 branchs must have been acquired independently of the spiral shell of 

 the Prosobranchs, and we should expect to find that the genera ex- 

 hibiting this feature are the most specialized Euthyneura, or in other 

 words, the most divergent from the general structure of the Strepto- 

 neura. This is not the case, however. The investigation of the 

 anatomy of Actceon by Bouvier, Pelseneer, and others, has shown that 

 this remarkable mollusc has not only a spiral shell, an operculum, a 

 gill in front of the heart, and a general asymmetry of the body, but 

 that it has also a typical Streptoneurous nervous system. 



In Scaphander, also, a genus with a less tightly twisted shell, 

 although there is no operculum and no actual twist of the visceral 

 nerve loop, the course which the nerves take and. the organs they 

 supply, indicate quite clearly that the loop has in this genus been 

 derived from a more primitive twisted condition, the right ganglion of 

 the visceral loop being above the crop and the left below it. Leaving 

 out of consideration several other intermediate forms, and passing on 

 to Aplysia and the Nudibranchs, we find that in those Opisthobranchs 

 with an untwisted shell or no shell at all, the visceral nerve loop is 

 perfectly straight and shows practically no sign of its primitive twisted 

 condition. 



The evidence derived from a study of the nervous systems of 

 these animals indicates, then, that the euthyneurous condition has 

 been arrived at by an untwisting of a more primitive condition, and 

 this is confirmed and supported by the fact that Actceon and Scaph- 

 ander, in which the visceral nerve loop is not completely untwisted, 

 are in other respects more closely related to the Streptoneura than 

 other Euthyneura, and by the fact that many larval Euthyneura have 

 a coiled shell. 



