THE LAMELLIBRANCHIA 



235 



(Fig. 215, c), and in exceptional cases a little ganglion at about the 

 middle of its length on either side, at the point of origin of 

 the branchial, pallial, or visceral nerves. These additional ganglia 

 occur in Dreissensia and in some other genera of Eulamellibranchia, 

 viz. the Unionidae, Cardium, Lutraria, Mya, and Solen. Each of 

 the three chief pairs of ganglia presents the same general 

 characters in the entire class, and special characters in certain 

 forms. 



The cerebro-pleural ganglia are supraoesophageal and are 

 situated above the buccal orifice ; generally on the posterior face 

 of the anterior adductor muscle when this exists (Figs. 230, 251, c.g). 

 In the Solenomyidae alone do they occupy a more posterior position 

 (Fig. 231, c.g). In the Protobranchia, and in Mactra cwallina and 

 Venus, the cerebro-pleural ganglia are attached to one another, but 

 in all other forms they stand 

 apart. They innervate the labial 

 palps, the anterior adductor 

 muscle, and the anterior part of 

 the mantle, and send fibres to the 

 osphradia and otocysts. 



The pedal ganglia are situated 

 in the mass of the foot at a 

 greater or less distance from the 

 cerebral centres ; they are always 

 attached to one another (Figs. 

 214, p.g- 215, 1>). When the 

 foot is atrophied they become 

 more and more reduced e.g. in 

 Teredo (Fig. 195, p.g), Ostraea, 

 Pecten (Fig. 215, C) and the 

 pedal connectives become very 

 short. 



The visceral ganglia are situ- 

 ated some distance behind the 



Tw-kcfpri' arMnpfrr YniiQflp in all 



posterior adductor muscle m an 



the Protobranchia (Fig. 231, V.g\ ganglion - pair ; b, pedal ganglion - pair ; o, 

 , . . T 11 \. c i i visceral ganglion - pair. (From Lankester, 



but in other Lamellibranchia they after Gegwibmr.) 

 are to be found on the ventral 



face of this muscle (Figs. 188, 218, 219, etc., r.g\ except in Tlirada, 

 in which they are in front of it, and in some highly specialised 

 forms in which they are again behind it, as, for example, in Pholas, 

 and particularly in Teredo, in which the posterior adductor is 

 shifted forward (Fig. 195, v.g). The visceral ganglia are generally 

 superficial, and barely covered by the tegumentary epithelium 

 (Figs. 188, 236), but in Lima they are somewhat deeply embedded 

 in the visceral mass. The two ganglia are primitively distant 



Fio. 215. 



Central nervous system. A, of Teredo; 

 of Anodonta; C, of Pecteit. a, cerebral 



