278 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



In some sub-genera they are united to the a1)domen from 

 this point to their anterior ends, but in Anodon and most 

 Unios they are free for a small part of their length, so that 

 there is a direct communication between the branchial and 

 cloacal chambers. 



k. The JVervous System. After the gills have been 

 separated from each other, as described in ^, the lower 

 surface of the posterior adductor muscle (Fig. 143, e) will 

 be seen near the posterior end of the body. 



1. Near the anterior edge of the muscle a pair of 

 orange-brown masses, the parieto-splanchnic ganglia 

 (Fig. 143 and 144, I) will be seen, covered by a trans- 

 parent layer of integument. Carefully dissect this oif, to 

 expose the ganglia and the nerves which run from them, 

 noticing : — 



(i.) A nerve which runs backwards to the rectum {m). 



(ii.) A pair of large pallial nerves, which run backwards 

 and outwards to innervate the edges of the mouth. 



(iii.) A pair of large branchial nerves, which run to the 

 gills. 



(iv.) A number of small nerves, which run forwards 

 and outwards from the ganglion to adjacent parts. 



(v.) Near the middle line a pair of much larger trunks, 

 the cerebro-visceral commissures (Figs. 143 and 144, k). 

 These can be traced forward for some distance, but more 

 anteriorly they pass into the substance of the abdomen 

 (ab), and cannot be traced without dissection. Carefully 

 dissect them out as far as the anterior edge of the abdo- 

 men. 



2. Each of them will be found to join a small cerebral 

 ganglion (Figs. 143 and 144, i) The two cerebral ganglia 

 lie at the sides of the mouth under the labial palpi. Each 

 gives rise to pallial nerves, which pass to the mouth ; 



