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 genex'al 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 branchia3 with 

 their nerves, and when these 

 are approximated on each side, 

 it is single ; when they are 

 wider apart, it is double {b). 

 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 (Jiff. 

 191) the labial ganglions (/) 

 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 (b). 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 Mytihis 



Mytilus eiiulis. 



