CONCHIFERA. 



391 



at the extremities ; these constitute the division to which Cuvier 

 has applied the name " Enfermes," on account of the very com- 

 plete union of the two sides of the mantle ; and from such forms 

 of CONCHIFERA the transition to the TUNICATA, described in 

 the last chapter, is by no means difficult. 



(425.) In animals circumstanced as the CONCHIFERA, it would be 

 vain to expect any high developement of the nervous system, or senses 

 of an elevated character : nevertheless, a few small ganglia are percep- 

 tible in different parts, and nervous threads of extreme tenuity are 

 seen to arise from them, and to be distributed in various directions. 



One pair of ganglia is, in the Dimyaria, easily distinguished, 

 occupying the ordinary position of the brain, namely above the oeso- 

 phagus. Hence is derived a supply of nerves to the sensitive labial 

 appendages, to the oral orifice, and other neighbouring parts. Two 

 other ganglionic masses, of larger size than the brains properly so 

 called, are placed near the posterior retractor muscle ; and a fifth 

 small ganglion, in those species provided with syphons, is found 

 in the vicinity of the breathing-tube, the muscular walls of which 

 receive nerves from this source. Fig. 181. 



In the Mono- 

 myaria the nervous 

 centres are still 

 more feebly deve- 

 loped, and the pos- 

 terior ganglia pro- 

 portionately smaller 

 than those found in 

 species possessed of 

 two adductor mus- 

 cles. 



(426.) No or- 

 gans of sense, other 

 than those already 

 noticed, are met 

 within any of the 

 Conchifera, except 

 in one remarkable 

 instance. In the 

 Scallops (Pecten) 

 the edges of the 

 mantle are studded with numerous pearl-like points, interspersed 



