TYPE M0LLTT80A. 



311 



Fig. 139.— Nervous System of 

 Fiona atlantica (after Bebgh from 

 Gegeneaur). 

 A — cerebro-pleuro-visceral gan- 

 B = pedal ganglion. [glion. 



C = buccal ganglion. 

 D = gastro-oesophageal ganglion. 

 e = pedal commissure. 

 e* = visceral commissure. 



to three such gauglia may be distinguished. In the figure of 

 the nervous system of Fiona (Fig. 139) the concentration of 

 the ganglia is well marked, but a 

 decided asymmetry is made evi- 

 dent in some forms by the exist- 

 ence of a single parietal ganglion 

 and of three visceral ganglia. In 

 Fiona, however, the ganglionic 

 concentration has been carried 

 still farther, and at the same time 

 by the suppression of the parietal 

 ganglion as a distinct mass of 

 cells an apparent symmetry has 

 resulted. 



With regard to many other 

 features of their anatomy con- 

 siderable variations are to be 

 found. Thus in some forms a 

 well-developed spirally-coiled visceral hump is developed, 

 while in others it loses its spiral arrangement, and in others 

 again is elongated in the direction of the foot and can hardly 

 be said to exist. So, too, with the occurrence of the shell, 

 mantle, and branchiae ; all are well developed in some forms, 

 but entirely absent in others. These peculiarities will be more 

 conveniently referred to in connection with the various groups, 

 and it is only necessary here to refer to another feature in 

 addition to those already given, which is common to all the 

 members of the order — i.e., the hermaphroditic character of 

 the reproductive gland. 



This forms part of the visceral mass and is usually com- 

 posed of numerous lobes, these again being divided into 

 secondary lobes or acini, the lining epitheliutn of which 

 give rise to both ova and spermatozoa. In some forms, 

 such as BvUa and Aplysia, both elements are formed in all 

 the acini ; but in others, such as Doris, Janus, Pteropoda, etc., 

 the epithelium of the terminal acini gives rise to ova only ; the 

 epithelium of the lobes, i.e., the central portions of the gland, 

 producing spermatozoa. Whether or not, however, there be 

 such a separation of the epithelium into male and female 



