826 MANUAL OF THE MOLLUSCA. 
mantle whatever. The eyes appear as minute black dots, 
immersed in the skin, behind the tentacles; they are well 
organised and conspicuous in the young, but often inyisible in 
the adult. The dorsal tentacles are laminated, like the antennz 
of many insects (Fig. 11, p.17); they are never used as 
organs of touch, and are supplied with nerves from the olfactory 
ganglia. The nervous centres are often conspicuous by their 
bright orange colour; they are concentrated above the 
cesophagus ; three pairs are larger than the rest, the cerebroid 
in front, the branchial behind, and the pedal ganglia at the 
sides. The cerebroid supplies neryes to the tentacles, mouth, 
and lips. 
The olfactory ganglia are sessile on the front of the cerebroid 
(in Doris), or situated at the base of the tentacles (in olis). 
The optic ganglia are placed on the posterior border of the 
cerebroid; the auditory capsules are sessile on the cerebroid, 
immediately behind the eyes, they contain an agglomeration of 
minute otolites, which are continually oscillating.* The buccal 
ganglia are below the cesophagus, united to the cerebroid by 
commissures, forming a ring; anterior to this a small ring is 
sometimes formed by the union of the fifth pair of neryes. The 
pedal ganglia (properly infra-cesophageal) are united laterally 
to the cerebroid and rarely meet below, but are united by com- 
missures which form (together with those of the branchial 
centres) the third ring, or great nervous collar. The branchial 
ganglia are united behind to the cerebroid, and sometimes 
blend with them; they supply the skin of the back, the rudi- 
mentary mantle, and the gills; beneath and sessile on their 
front border is the single visceral ganglion. Besides this eucito- 
motory system (which includes the great centres, or brain, and 
the neryes of sensation and voluntary motion), the nudibranchs 
possess a sympathetic system, consisting of innumerable minute 
ganglia, dotted over all the viscera, united by nerves forming 
plexuses, and connected in front with the buccal and branchial 
centres.t 
The digestive organs of the Nudibranchs present two remark- 
able modifications: in Doris and T'ritonia the liver is compact 
* The auditory capsules of other Mollusca (excepting the Nucleobranchs) are 
attached to the posterior side of the pedal (sub-cesophageal) ganelia, 
7 The sympathetic system supplies nerves to the heart and other organs which are 
independent of the will, and not ordinarily susceptible of pain; they are called 
“organic” nerves, as all the vegetative functions depend onthem. Its existence in the 
Mollusca was first clearly demonstrated by MM. Hancock and Embleton., The ezcito- 
motory system of the Mollusca corresponds with the cerebro-spinal system of the 
vertebrata, 
