284 LECTURE XXI. 



The returning veins of the body form a remarkable plexus at the 

 base of the gills, near the pericardium, which assumes the form of a 

 distinct glandular organ in the higher bivalves. The secretion of tiiis 

 venous body abounds with calcareous particles, and the gland was 

 called by Poli the secreting organ of the shell : it is shown at 

 fig. 118. r. Modern analysis has detected a large proportion of uric 

 acid in the peritoneal compartment enclosing this venous plexus, and 

 has thus determined it to be the renal organ. 



The nervous system advances in a regularly proportional degree 

 with the complexity of the general organisation, and especially with 

 the muscular system : the ganglion upon the posterior adductor, 

 which is most conspicuous in the oyster, is the largest and most 

 constant in all other bivalves : it supplies the branchiae with their 

 nerves, and when these are approximated on each side, it is single ; 

 when they are wider apart, it is double. It is called, therefore, the 

 branchial ganglion, but it distributes an equal share of nerves to the 

 posterior and dorsal parts of the mantle. 



In the common mussel the labial ganglions may be distinguished 

 by their yellow colour at the base of the labial processes. They are 

 connected by a short transverse nervous chord, passing above or in 

 front of the mouth. From each of the ganglions two principal nerves 

 are given off, one passing forwards to the anterior adductor, the other 

 backwards along the base of the foot and the visceral mass to the 

 posterior adductor. At a short distance from the labial ganglion, 

 this latter nervous chord sends off a branch, which communicates 

 with its fellow by means of a bilobed ganglion, situated at the anterior 

 pai't of the base of the foot. This pedial ganglion and the labial and 

 branchial ganglions just described, constitute the principal centres of 

 the nervous system in the common bivalve here dissected with so 

 much elaborate minuteness by Mr. Goadby. The pedal ganglion 

 distributes nerves in one direction to the retractors ; in another to the 

 substance of the foot. The branchial ganglions send off" nerves 

 which are distributed principally to the posterior pair of the breathing 

 organs, and two large nerves which diverge as they pass over the 

 adductor muscles to proceed to the base of the tentacular processes 

 guarding the posterior lobes of the mantle : these continue along 

 the margin of each lobe of the mantle until they meet and anas, 

 tomose with corresponding branches, which are continued over the 

 anterior adductor muscles from the labial ganglions. Cuvier ac- 

 curately describes this important feature in the nervous sj'stem of the 

 bivalves. The marginal pallial nerve is not, however, simple, but 

 consists rather of a sei-ies of elongated loops.* 



* Many other particulars of the nervous system of the Mytilus, demonstrated in 



