TYPE MOLLUSC A, 335 



each side, it is to be regarded as a cerebro-pleural ganglion. 

 From each of these a pedal connective passes downwards into 

 the foot to terminate in a paired pedal ganglion (pg), and a 

 second strong connective passes backwards on each side of 

 the base of the foot to terminate in a large ganglion (vi), sit- 

 uated below the rectal portion of the intestine and frequently 

 in close proximity to the posterior adductor muscle, and which 

 from its relations is evidently to be regarded as representing 

 both the parietal and the visceral ganglia of the Gasteropods 

 and hence may be termed the viscero-parietal ganglion. 



The sense-organs are of essentially the same nature as in 

 the Gasteropods. Tactile cells exist scattered over the sur- 

 face of the body, and are especially numerous in certain lo- 

 calities, as upon the siphons when these are present. A pair 

 of osphradia are also present situated above the viscero-parie- 

 tal ganglion close to the insertion of the bases of the gill- 

 plates into the side of the body ; and imbedded in the tissues 

 of the foot, usually in close proximity to the pedal ganglia, 

 though innervated by the cerebro-pleural, are a pair of oto- 

 cysts having the usual structure (see p. 283). In a number of 

 forms paired elevations, evidently of a sensory nature, have 

 been found in the neighborhood of the inner ends of the 

 siphons, or on the sides of the body a little in front of the 

 anus ; the function of these is doubtful, though it has been 

 suggested that they are olfactory. 



Eyes are present in a number of forms and present various 

 degrees of complexity. In some cases a perception of sudden 

 variations in the intensity of light is present, as in the siphons 

 of some forms, without any distinct optic sense-organs being 

 developed. Sensory and pigment cells are present, however, 

 and may be regarded as forming a diffuse optic organ. No 

 eyes occur upon the head, nor are tentacles developed in any 

 of the Pelecypods, but large numbers of eyes are developed 

 upon the edge of the mantle of many forms, such as Pecten 

 and Spondylus. These eyes may be simple depressions of the 

 mantle-margin, the bottom of the depression being lined with 

 pigmented and sensory cells, a cuticle of varying thickness cov- 

 ering this retinal surface. Another form of eye (Fig. 150) also 

 occurs upon tentacular processes which presents an arrange- 



