326 MANUAL OF THE MOLLTJSCA. 



mantle wtate-^er. The eyes appear as minute black dot?, 

 immersed in the sldn, behind the tentacles; they are well 

 organised and conspicuous in the young, but often invisible in 

 the adult. The dorsal tentacles are laminated, like the antennae 

 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 

 oesophagus ; three pairs are larger than the rest, the cerebroid 

 in front, the hranchial behind, and the pedal ganglia at the 

 sides. The cerebroid supplies nerves to the tentacles, mouth, 

 and lips. 



The olfactory ganglia are sessile on the front of the cerebroid 

 (in Doris) y or situated at the base of the tentacles (in y^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 oesophagus, 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 nerves. The 

 pedal ganglia (proj)erly infra-oesophageal) 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 thii'd ring, or great nervous collar. The branchial 

 ganglia are united behind to the cerebroid, and sometimea 

 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 excite- 

 motory system (which includes the great centres, or brain, and 

 the nerves 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 Tritonia the liver is compact 



* The auditory capsxiles of other Mollusca (excepting the Nucleobranchs) are 

 attached to the posterior side of the pedal (sub-oesophageal) gangUa. 



t The sympathetic system supplies nerves to the heart and other organs which aro 

 independent of the will, and not ordinarily susceptible of pain ; they are called 

 «' organic" nerves, as aU the vegetative functions depend on them. Its existence in the 

 Mollusca was first clearly demonstrated by MM. Hancock and Embleton. The excite 

 motory system of the Mollusca corresponds with the cerebrospinal system of tho 

 vertebrata. 



