44 PHYSIOLOGICAL SERIES. 



yellow in the specimen). They not only form a layer of 

 some thickness upon the three pairs of ganglia and the 

 root of the branchial nerve as in other Pelecypods, but cover 

 the outer and dorsal surfaces of the oerebro-viflceral 

 connectives and the cerebral and visceral commissure-. 



Pelseneer, Arch, de Biol., t. xi. 1891, p. 166. 



Drew, Quart. Journ. Micro. Soc., vol. xliv. 1901, p. 373. 



D. 41. The nervous system of an Edible Mussel (Mytilus 



isolated. The ganglia are small the pedal and visceral 

 of approximately equal size, the cerebral slightly smaller. 

 The latter lie on either side of the mouth, united by a long 

 pr3oral commissure. From the posterior extremity of 

 each a single cord is given off, which shortly divides to 

 form the cerebro-pedal and cerebro-visceral connectives. 

 The pedal ganglia lie between the viscera and the foot ; 

 they are closely applied to one another, and each gives off 

 two main nerves a large one from its lateral border to the 

 pedal muscles, and a smaller posterior one to the byssus 

 gland. The visceral ganglia lie some distance apart upon 

 the antero-ventral surface of the posterior adductor, united 

 to one another by a slightly ganglionic commissure. Each 

 ganglion gives off two principal nerves, an anterior 

 ganglionic branchial nerve and a large posterior trunk that 

 supplies the mantle and posterior adductor. 



D. 42. Two specimens of the nervous system of a Scallop 

 (Pectcn ma.rimus) one isolated, the other >ho\vn within 

 the body from the left side. The remarkable perfection of 

 tin- sense-organs on the mantle-border of the Scallops, and 

 the large size and energy of their adductor muscle, are 

 accompanied by a corresponding development of the visceral 

 ganglia, the centres from which the-e pails are mainly 

 innervated. The ganglia are completely fused in the 

 mid-line, and form a rectangular ma>s Htiiafed upon the 

 ventral .-urfaee of the adductor, near the centre of the animal. 

 Kadi anterior corner receives one of the cerebro-visceral 

 connectives; external to these arise the branchial ner\- -. 

 followed, alon^ each lateral margin of the ganglion, by 

 a serif*> ol* lateral mantle-nerves, di.Mriluited to the middle 



