GASTEROPODA. 189 



satisfactory account, however, has been published of any except the European, 

 and especially the British species, which form the subject of an admirable 

 monograph by Messrs. Alder and Hancock, in the transactions of the Ray 

 Society. They require to be watched and drawn whilst living and active, 

 since after immersion in spirits they lose both their form and colour. In 

 some the back is covered with a cloak or mantle ( ?,) which contains calcari- 

 ous spicula of various forms, sometimes so abundant as to form a hard shield- 

 like crust.* The dorsal tentacles and gills pass through holes in the cloak 

 somewhat like the " key-hole " in Flssurella. In others there is no trace of 

 a mantle whatever. The eyes appear as minute black dots, immersed in the 

 skin, behind the tentacles ; they are well organized, and conspicuous in the 

 young, but often invisible in the adult. The dorsal tentacles are laminated, 

 like the antennse of many insects (fig. 11, p. 23) ; 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 branchial 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) 

 or situated at the base of the tentacles (mjEolis). 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 ag- 

 glomeration of minute otolites which are continually oscillating.f The luc- 

 cal 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 5th pair of nerves. The pedal ganglia (properly infra- cesopha- 

 geal) are united laterally to the cerebroid and rarely meet below, but are 

 united by commissures which form (together with those of the branchial 

 centres) the 3rd 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 rudimentary mantle, and the gills ; beneath, and 

 sessile on their front border is the single visceral ganglion. Besides this 

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

 nerves of sensation and voluntary motion), the nudibranches possess a sym- 

 pathetic 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.:}: 



* According to Mr. Huxley, the " cloak" of the Dorids is not the equivalent of 

 the mantle, but " has more relation to the epipodium." 



t The auditory capsules of other Mollusca (excepting the Nucleobranches) are 

 attached to the posterior side of the pedal (sub-cesophageal) ganglia. 



J 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 " or- 

 ganic " nerves, as all the vegetative functions depend on them. Its existence in the 



