504 CONCHIFEKA. 



are perpetually rushing through the extremities of each siphon, caused 

 by the rapid action of cilia placed within ; and the streams thus pro- 

 duced not only form a provision for constantly changing the water in 

 which the branchiae (fig. 251, g) are immersed, but forcibly convey 

 floating molecules to the aperture of the mouth, which is situated in 

 the position indicated in the figure by . the letter A, and thus supply 

 abundance of nutritive materials that could, apparently, in animals so 

 destitute of prehensile organs, have been procured by no other con- 

 trivance*. 



(1325.) The last family of this class includes those species which, 

 like the Pholas and Teredo, bore in stone or wood, or, like the Sohn, 

 penetrate deeply into the sand. In such, the mantle is prolonged into 

 terminal tubes of great length, and their shells remain always open at 

 the extremities : these constitute the division to which Cuvier has ap- 

 plied the name " Enfermes," on account of the very complete union of 

 the two sides of the mantle ; and from such forms of CONCHIFEKA the 

 transition to the TUNICATA, described in the last chapter, is by no 

 means difficult. 



(1326.) In animals circumstanced as the CONCHIFERA, it would be 

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

 an elevated character ; nevertheless a few small ganglia are perceptible 

 in different parts, and nervous threads of extreme tenuity are seen to 

 arise from them and to be distributed in various directions. 



(1327.) One pair of ganglia, in the Dimyaria, is easily distinguished, 

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

 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 siphons, is found in the vicinity 

 of the breathing-tube, the muscular walls of which receive nerves from 

 this source. 



(1328.) In the Monomyaria the nervous centres are still more feebly 

 developed, and the posterior ganglia proportionately smaller than those 

 found in species possessed of two adductor muscles. 



(1329.) No organ of sense, other than those already noticed, is met 

 with in any of the Conchifera, except in one remarkable instance. In 

 the Scallops (Pecteri) the edges of the mantle are studded with nume- 

 rous pearl-like points, interspersed among the retractile tentacula placed 

 around its circumference. These, which are represented in the figure 

 of Pecten already given (fig. 247), are considered by Polif to be so 



* The parts represented in the above figure (fig. 251) which are not particularly 

 pointed out in the text are the anterior adductor muscle (c), the posterior adductor 

 muscle (d), the elastic ligament of the hinge (e), and the largely-developed foot (/). 



t Testacea utriusque Sicilian, eorumque Historia et Anatome : 3 vols. fol. 



