POLYCERA. 



the other (6), passing forward, supplies the same in front, and also the tentacular filaments. 

 The right branchial gives off from the under surface an additional nerve in connexion with 

 the branchial collar, which shortly enlarges into a small round ganglion— the visceral (e) : 

 this sends two or three nerves (17, 18) to the viscera. The left branchial ganglion also 

 gives off in connexion with the collar another visceral nerve (16). The pedial ganglia 

 {c, c) are of an oval form, and are placed at the outer margins of the branchial, to which, and 

 to the cerebroid, they are attached by very short commissures. They each give three nerves 

 (9, 10, 11) to the foot, and the left ganglion a small one (8) to the side of the body. A 

 similar nerve has not been observed on the right side, though it probably exists. These 

 ganglia are united below the oesophagus by a long collar or commissure, (i) which is composed 

 of two filaments. The branchial are likewise united below the alimentary tube ; the cord (A), 

 which is fine, leaving the ganglia in connexion with the origin of the visceral nerves. These 

 two commissures run across in union with each other, forming, as it were, but one great 

 oesophageal collar. 



The buccal ganglia (/) are of an oval form ; they are united across the median line by a 

 distinct commissure, and to the cerebroids by long, slender cords {g,g). These ganglia give 

 nerves in the usual way to the buccal organ, tongue, and oesophagus. The gastroesopha- 

 geal ganglia have been observed only in P. ocellata. 



P. Lessonii and P. ocellata exhibit no material modification in the nervous system : the 

 cerebroids, however, are more rounded in the latter ; and in the former are broad in front and 

 pointed backwards, approximating in form to those of Doris tuberculata. The right branchial, 

 too, of both species gives off a nerve in front, which goes to the male generative organ. 

 There can be little doubt of the existence of a similar nerve in P. quadrilineata, but it has 

 hitherto escaped observation. The visceral ganglion has not been noticed in P. Lessonii. 



The eyes are rather large, and are as completely organised as in Doris; in P. quadri- 

 lineata they are each supplied with a short optic nerve (5) arising from a minute elliptical 

 ganglion appended to the outer margin of the cerebroids. The auditory capsules contain 

 numerous otolithes, and are attached to the under side of the outer margin of the same gan- 

 glia immediately behind the eyes. These have been examined only in P. Lessonii. 



Vibratile cilia cover the whole surface of the body, including the tubercles, as well as the 

 tentacles, gills, and branchial processes. 



The skin is comparatively thin and delicate, having a few soft tubercles on the surface. 

 The imbedded spicula are rather sparingly scattered ; they are stout, linear, and generally 

 tuberculated, and bent a little in the centre. 



