NERVOUS SYSTEM. INVEKTEBRATA. 55 



off from the right visceral ganglion there is a small round 

 ganglion ; this underlies a sensory pit (probably olfactory) 

 situated close to the respiratory orifice. The central 

 ganglia are considerably tabulated. Blue paper has been 

 placed beneath the visceral loop in both specimens. 



0.0. 1305 A b. 

 Lacaze-Duthiers, Arch. Zool. Exp., t. i. 1872, p. 437. 



D. 55. Two specimens of the nervous system of a Sea-Hare 

 (Aplysia punctata), seen from the dorsal aspect, isolated 

 (upper specimen), and in situ. The several ganglia of the 

 circum-03sophageal ring are separate, definite in outline, 

 and of moderate size. The cerebral ganglia lie close together 

 above the oesophagus, on either side of which they are 

 united by a pair of short connectives to the pedal and pleural 

 ganglia. Each pleural ganglion lies slightly behind and 

 below the pedal ganglion of the same side, joined to it by 

 a very short connective. The pedal ganglia are united by 

 two commissures one short and stout passing directly from 

 ganglion to ganglion, the other longer and more delicate. 

 The pleural ganglia give rise to a long untwisted (ortho- 

 neurous) visceral loop, that extends backwards through the 

 cavity of the body to the pericardium. Here it is completed 

 by a large bilobed ganglion, from which nerves are given 

 off to the body-wall, generative organs, and gill. The 

 branchial nerve arises from the upper of the two lobes, and 

 at the base of the gill forms a small round ganglion that 

 underlies a special sense-organ (osphradium) . The chief 

 nerve given off from the lower lobe supplies the genital 

 organs. Although the visceral loop is essentially ortho- 

 neurous, it shows a variable but distinct tendency towards 

 a streptoneurous twist. When seen from above its left arm 

 appears in the posterior part of its course to lie directly 

 beneath or even in some cases slightly to the right of the 

 right arm. This partial streptoneury is interesting in view 

 of the unmistakably twisted loop of another Opisthobranch 

 (Actceon). The nerve distribution resembles that in other 

 Gastropods ; the sense-organs of the head and the integu- 

 ment around the mouth are supplied from the cerebral 

 ganglia ; the pleural nerves innervate the anterior parts of 



