NERVOUS SYSTEM. INVERTEBRATA. 43 



concentration, and complexity of structure, depending upon 

 the perfection of the mantle sense-organs or the formation of 

 siphons. 



PELECYPODA. 

 Rawitz, Jena. Zeits., Bd. xx. 1887, p. 384. 



D. 40. A wax model of the nervous system of Nucula nucleus, 

 X 25. The central system, as in other Pelecypods, consists 

 of three pairs of ganglia united to one another by com- 

 missures and connectives. The cerebral ganglia lie in 

 front of the mouth united by a short commissure ; each 

 gives off three nerves that respectively innervate the 

 anterior adductor, labial palps, and anterior part of the 

 mantle. Posteriorly, the cerebral ganglia gradually di- 

 minish in thickness and pass insensibly into a pair of long 

 cerebro-visceral connectives, by which they are united to 

 the visceral ganglia. The latter are small and laterally 

 separate ; they lie ventral to the gut at some little distance 

 in front of the posterior adductor united by a definite 

 commissure, and innervate the gills, posterior adductor, 

 and posterior part of the mantle. The cerebro-pedal 

 connectives arise from the inner ventral surface of each 

 cerebral ganglion by two roots, that remain separate for a 

 fourth of the distance to the pedal ganglion. The double 

 origin of these connectives from each cerebral ganglion 

 has been thought to indicate that the latter is a compound 

 structure consisting of two ganglia comparable to the 

 cerebral and pleural of the Gastropod. But it may be, 

 that the posterior root of the cerebro-pedal connective is 

 the proximal part of the otocystic nerve running free for 

 the first part of its course and not completely united with 

 the cerebro-pedal connective as in most other Pelecypods. 

 It is suggestive in this connection that in Solenomya (another 

 of the Protobranchia) the otocystic nerve is entirely free 

 from cerebral ganglion to otocyst. The pedal ganglia lie 

 in the substance of the foot closely united together by a 

 double commissure; they innervate the pedal muscles. The 

 nervous system shows its low organisation by the diffuse 

 distribution of ganglion-cells upon its surface (indicated by 



