96 On the genus Onchidium. [No. 2, 



with the intestinal and the generative organs. The arterial blood 

 is white, and the corpuscles very minute and of an oval shape. 



Nervous system and organs of sensation. (See fig. 5, pi. xiv.) 

 The principal ganglion which is a thick white ring, lies immediately 

 behind the head j a portion of the aorta and the alimentary canal 

 passing through it. This ganglion gives up numerous branches 

 laterally to the base of the eye-pedicles, the tentacular rims and buccal 

 appendages. One thick branch, subsequently dividing, issues below 

 and supplies the head, some of its small nervous threads uniting into 

 a small ganglion between the oral appendages. Another very thick 

 branch also issues from the central ganglion below, and is directed 

 backwards, accompanying the alimentary canal. It divides at the 

 digestive organs in two branches, one supplying these and the other 

 the generative organs. Besides these, there issue from the central 

 ganglion five long threads on each side, two giving the requisite 

 number of nerves to the foot and four (or 8 altogether) to the mantle. 

 They appear, however, to be connected with the other nervous 

 branches of the intestines by numerous very fine threads. 



From the generic characteristic which I have previously given, 

 it will be seen that I have made the distinction between eye- 

 pedicles and tentacles. This verbal distinction is, I believe, in 

 most of the Gastropods, an essential one and it is, for instance, 

 not correct to speak in the HelicidxE of four tentacles, for they do not 

 all serve the same purposes. Strictly speaking, there is only one pair 

 of each, two tentacles and two pedicles. The presence of only one 

 pair of tentacles, — actually the eye-pedicles, — has been pronounced as 

 a peculiarity of the Onchidia and was used as an important distinc- 

 tion from the genus Vaginulus. The Oncliidia possess, however, 

 beside the pair of prolonged pedicles, a pair of true tentacles, which 

 appear as thickened rims on the upper surface of the buccal append- 

 ages. Thus the distinction from Vaginulus, which has the tenta- 

 cles free and bilobed, is in this point only a gradual one of develop- 

 ment. 



When the mantle of an Onchidium is dorsally cut open, and the . 

 internal organs exposed, the dark pedicles are seen to be attached 

 laterally to the mantle, reaching with their bases beyond the li^ad 



