1869.] On the genus Onchidium. 1)7 



(seepe in fig. 5, pi. xiv ; and the figure between 1 and 1 a). The base 

 of each is flattened, white, cartilaginous, intimately connected with the 

 muscular tissue of the mantle in this place ; above the base numerous 

 nerves enter to it, and the trunk of the pedicle becomes hollow, 

 more cylindrical and soft. The small, black eye is situated eccentri- 

 cally near the tip, which is pointed, angularly bent and attached by 

 a strong muscle to the internal side of the outer skin (tp } in fig. 5) 

 of each pedicle. The muscle then bends backward, and joins the trunk 

 of the pedicle about one-third or one-fourth of the length distant from 

 the tip. The external cover of the pedicle, is formed by the soft skin, 

 in the fold between the head and the mantle. 



This organisation of the pedicles fully agrees with that of the 

 Helicidje in general, and makes it perfectly clear that the idea 

 as to the non-retractibility of the pedicles in Onchidium cannot 

 be retained. In all the species of Onchidium, of which I have 

 observed live animals, I found the pedicles to be almost entirely 

 retractile, but it is not usual that an animal, unless strongly 

 irritated, does retract them fully, because the mantle which covers 

 the head gives, as a rule, sufficient protection to them. Whenever 

 specimens are, however, put in spirit, it is a common case that the 

 strongly muscular mantle and the disc of the foot shrink more ra- 

 pidly than the soft skin between them, and the head with its pedicles, 

 and tentacles and buccal appendages is consequently easily pressed 

 out. Thus the examination of specimens in spirit, evidently seems 

 to have given ground to the idea, that the pedicles in Onchidium 

 are not retractile. This observation appears to have been sup- 

 ported by the existence of two indentations, which are formed in 

 the edge of the mantle above the pedicles, when the animal moves 

 about. Occasionally these indentations, or grooves, are traceable 

 for some time even after the death of the animal, but they are 

 by no means permanent, and constantly change in live specimens. 

 Whenever the animal retracts its head, and covers it from above with 

 the mantle, and from below with the front edge of the foot, the in- 

 dentations perfectly disappear in each such case. 



The true tentacles are, as already noticed, in their entire length 



grown to the upper surface of the buccal appendages, and generally 



| are with their external terminations connected with the extreme 



13 



