132 COMPARATIVE ANATOMY CHAP. 



On each side of the cerebral ganglion, a nerve rises, which runs to 

 a ganglion below the pharynx and behind the radular sheath, this is 

 the sublingual ganglion ; this latter is united with the corresponding 

 ganglion on the other side by a short transverse commissure. These 

 sublingual ganglia probably correspond with the buccal ganglia of 

 Chiton. 



Dondersia is specially noteworthy because distinct ganglionic swellings occur at 

 regular intervals along the pedal cords ; this is particularly marked in the anterior 

 part of the body. The equally regularly repeated transverse commissures joining the 

 pedal cords, and the connectives between the pedal and visceral cords, start from 

 these distinct ganglia. 



In Lepidomenia hystrix, one ganglion occurs posteriorly and one anteriorly in 

 each longitudinal trunk (whether pleurovisceral or pedal), and each is connected 

 with a similar ganglion of the opposite side by a transverse commissure. 



In Neomenia and Chcetoderma, no connectives between the visceral and pedal 



FIG. 112. Nervous system of Proneomenia Sluiteri (original drawing by J. Heuscher). 

 1, Cerebral ganglia ; 2, pleurovisceral cords ; 3, 4, 5, posterior ganglia of the pleurovisceral cords ; 

 6, sublingual ganglia; 7, anterior pedal ganglia; 8, right pedal cord; 9, left pedal cord; 10, 11, 

 strong posterior commissures between the pedal cords ; 12, anterior pedal commissure ; 13, sub- 

 lingual commissure. 



cords have been observed, and, so far as is at present known, in Chcetoderma, the 

 commissures between the pedal cords are also wanting. Further, in Cha-toderma, 

 the visceral and pedal cords of each side unite together posteriorly to form one 

 single cord, which becomes connected with the similar cord on the other side by a 

 transverse cord which runs over the cloaca. 1 



B. Gastropoda. 



The nervous system of the Gastropoda is of great interest to the 

 comparative anatomist on account of the crossing of the pleurovisceral 

 connectives in the Prosobranchia, which will be further described in this 

 section. 



The nervous system of this class consists typically of those parts 

 which we have already mentioned in our scheme of the organisation of 

 the Mollusca, viz. : 



1 For further details see Simrotli's new edition of Bronn's Klassen und Ordnungen 

 des Thier-reiches, vol. iii. 



