NUDIBRANCHIATA. 



409 



close to, or fused with, the cerebral, which are themselves closely approximated. 

 The visceral commissure is short and possesses but few ganglia. In Tylodina 

 alone is there an osphradium and osphradial ganglion. 



Fam. 1. Umbrellidae. Visceral sac and external shell in the form of a flat 

 cone ; foot very thick. Umbrella Lam. ; Tylodina Rafinesque. 



Fam. 2. Pleurobranchidae. Shell internal or absent ; anterior tentacles 

 forming a frontal velum ; spicules in the mantle ; foot flat. Pleurobranchus 

 Cuv., internal shell ; Pleurobranchaea Meckel (Fig. 323), no shell ; Neda Adams. 



Tribe 2. NUDIBRANCHIATA. 



Opisthobranchs without shell in 

 the adult state. 



There is no ctenidium or os- 

 phradium ; the nervous system is 



Br 



FIG. 323. Pleurobranchaea Meckdii (regne 

 animal). ' Br ctenidium ; F posterior 

 tentacle (rhinophore) ; R proboscis ; P 

 penis. 



xi--- 



Fio. 324. Nervous system of Tritonia hombergi 

 (after Pelseneer), dorsal view x 15. I cerebral 

 ganglion; II tentacular, III optic nerve; IV 

 pleuropedal commissure ; V penial nerve ; VI 

 parapedal commissure ; VI' visceral commis- 

 sure ; VII abdominal ganglion and genital 

 nerve; VIII stomatogastric commissure; IX 

 buccal, Xgastro-oesophageal ganglion ; XT pedal, 

 XII sub-cerebral commissure ; XIII pedal gan- 

 glion ; XTFcerebro-pedal commissure ; XVoto- 

 cyst ; XVI pleural ganglion. 



much concentrated, and the gan- 

 glia are generally aggregated, and 

 more or less fused together, on the 

 dorsal side of the oesophagus ; but 

 the different commissures visceral, 

 pedal, and buccal are distinct. 



There are four suboesophageal commissures the visceral, the pedal, the para- 

 pedal (a second thin commissure between the pedal ganglia), and the subcerebral 

 (Fig. 324). In addition to these there is the stomatogastric system, which 

 constitutes another commissure completed ventral to the oesophagus. The 

 nerves springing from the visceral commissure are always unsymmetrical and 

 are exclusively genital and reno-pericardial. 



The pedal commissure is generally long, but it is short in Fiona, Ancula, 



