36 



jection, containing the greater part of the digestive and 

 reproductive viscera. Then two great lines of descent were 

 produced, one leading to the Pteropoda and Cephalopoda, the 

 other to the Gastropoda. 



Following up the last of these we find that a large 

 shell became developed as a protection to the visceral mass, 

 and that (possibly, as Lankester suggests, as a result of 

 the weight of the shell falling on one side) the visceral dome 

 became twisted round spirally to the right, so that the anus 

 which was originally posterior came to be placed first on the 

 right side of the body, and then anteriorly above the head. 

 The whole visceral mass has also in most cases come to be 

 more or less coiled spirally, and the shell covering it has taken 

 the same form. By these changes the bilateral symmetry was 

 entirely destroyed. The ctenidia and the nephridia and other 

 paired organs shared in the torsion of the visceral mass, and, 

 in most cases, became unequally developed on the two sides 

 in consequence. In many cases the ctenidium and nephri- 

 dium, which were originally on the left side of the anus, 

 and which, after the changes, came to be upon the right 

 side, have become atrophied. 



There is reason to believe that these ancestral Gastropods 

 split into two series, in the one of which the visceral nerve 

 loop, with the visceral ganglia, became implicated in the 

 rotation of the visceral mass, the result being that the loop 

 was twisted into a figure of eight, and the ganglia changed 

 sides ; while in the other series, possibly on account of the 

 deeper position of the nerves in the body, they were not 

 affected by the other changes. For the first of these series, 

 in which the nerves cross, Spengel has proposed the name 

 Streptoneura ; and for the second, in which the visceral 

 nerves remain straight and unaltered, Euthyneura. 



The primitive Streptoneura divided into two series : 

 the Zygobranchiata {Patella, Haliotis, &c.) in which the 



