510 



LECTURE XXT. 



channels in the substance of the foot, thence extending into the 

 lobes of the mantle, and into a part of the visceral mass : by this 

 provision for the admission of water the foot can be swollen out, like 

 a sponge, and made to exceed the capacity of the shell. The rela- 

 tions of this aquiferous system to the sanguiferous one, are not 

 satisfactorily determined. The fine jets of water expelled from the 

 foot, and the border of the mantle, when a Solen is suddenly removed 

 from the water, are from the aquiferous canals. 



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 (Z»). 

 It is called, therefore, the . 

 branchial ganglion, but it dis- 

 tributes an equal share of 

 nerves to the posterior and 

 dorsal parts of the mantle. 

 In the common muscle (^fig. 

 191) the labial ganglions {V) 

 may be distinguished by their 

 yellow colour at the base of 

 the labial processes. They 

 are connected by a short trans- 

 verse nervous chord (c), pass- 

 ing above or in front of the 

 mouth. From each of the 

 ganglions two principal nerves 

 are given off, one (a) passing 

 forwards to the anterior ad- 

 ductor, the other {d) back- 

 wards along the base of the foot and the visceral mass to the poste- 

 rior adductor, where it joins the branchial ganglion (i). At a short 

 distance from the labial ganglion this latter nervous chord sends 

 off a branch (e), which communicates with its fellow by means of a 

 bilobed ganglion, situated at the anterior part of^ the base of the 

 foot. This pedial ganglion ( p) and the labial and branchial ganglions 

 constitute the principal centres of the nervous system in the Myt'dus 



Mytilus eduliii. 



