ON THE NERVOUS SYSTEM IN PELECYPODA. 



45 



system) the central nervous system, ignoring the parts concerned iu 

 the innervation of the buccal mass, consists essentially of an elongated 

 loop (the cerebro-pallio-visceral loop) that encircles the bodj' in a 

 position just above the branchial furrow, and innervates the head, 

 mantle, gills and viscera, and of two pedal cords in the foot, 

 connected in front with the cerebral portion of this loop, and united 

 irregularly by commissures. The cerebro-pallio-visceral loop has 

 the same structure and similar relations to the body (except for 

 its union posteriorly above and not below the gut) as the loop in 

 Nucula formed by the cerebral and visceral ganglia with their com- 

 missures and the cerebro-visceral connectives. Both are eanglionic 



CP. C<jr» 



'^.(jrm. 



Fig. III. — Lono^itudiual section of the pedal ganglia of Nueula mieleus. c.p.con. 

 cerebro- pedal connective ; p. com. pedal commissures ; p.n. pedal nerve. 



throughout, both have the same position, and both innervate the same 

 organs and regions of the body. In fact, they are without question 

 homologous structures. The differences that do exist show that the 

 loop in Nueula is somewhat specialised — the ganglionic matter is 

 concentrated principally towards the anterior and posterior regions of 

 the loop, forming delinite though not sharply limited cerebral and 

 visceral ganglia. These develop independently of the rest of the 

 loop, although their connectives by their mode of development and 

 adult structure show clearly enough that they once formed part of 

 a continuous loop of ganglionic matter, now in process of replace- 

 ment by connective fibres. Tlie anterior and posterior concentration 

 of ganglionic matter in this portion of the nervous system of Pelecypods 

 is doubtless correlated to some extent with the special localisation of 

 organs, characteristic of the class — as, for instance, the adductor 

 muscles (derivatives of the pallial musculature), one at either end of 



