410 



MOLLUSCA. 



and most Elysioidea, in which the pedal ganglia are approximated ventral to 

 the oesophagus. 



The parapedal commissure is homologous with the same structure in Tecti- 

 branchs, but it gives off no nerves. The relation of the aorta to these 

 commissures varies, as it does in the Tectibranchs. The dorsal appendages 

 or cerata are innervated from the pleural ganglia, showing that these structures 

 are homologous and of a pallial nature. The penis is of a pedal nature, being 

 innervated from the pedal ganglion as in the majority of Streptoneura. 



The hermaphrodite gland has male and female acini, which however 

 communicate. The animals are protandrous. 



The characters of specialization of th%N"udibranchs are as follows : 



(1) Return to external symmetry. This was indicated in Tectibranchs, but 

 it is carried further in Nudibranchs, where there may be complete external 

 symmetry as in the Doridioidea with median anus, and even internal symmetry 

 in many of the organs (heart, kidney, liver), but never in the generative 

 apparatus. In the Tritonioidea this internal symmetry is not found. 



(2) Displacement of the nervous system behind the buccal mass. 



(3) Concentration of the central nervous system dorsal to the oesophagus. 

 The pedal ganglia are approximated to the cerebral, and the pleural are fused 

 with the cerebral except in Tritoniidae. But as already mentioned in Fiona, 

 Ancula, and in most Elysioidea, we find the pedal more or less approximated 

 ventrally to oesophagus, and distinct ganglia on the short visceral commissure. 

 In the presence of distinct ganglia on the visceral commissure, often three in 

 number, the Elysioidea are more primitive than other Nudibranchs. 



(4) Reduction of the number of teeth of the radula (even to a single tooth 

 for each transverse row). 



(5) Diffusion of the liver. 



(6) Triaulic condition of genital ducts. 



The Tritoniidae, which are Nudibranchs because they are without ctenidium, 

 osphradium, mantle-fold, shell, and because they have cerata, are without any 

 of these features of specialization ; except the dorsal grouping of the cerebral, 

 pedal, and pleural ganglia, which are, however, distinct. But even in the 



EOUD 



'ELYSID 



If we had another dimension we could connect the Elysid group and Janus to show the 

 common presence of cerata. 



