142 
THE VOYAGE OF H.M.S. CHALLENGER. 
hinder end (fig. 14&) of this the fascicles of a strong retractor muscle become broader 
and are fastened ; this last has several points of origin, apparently united with the above- 
mentioned retractor, on the under side of the anterior portion of the prostatic coil of the 
vas deferens ; the flat belly of the muscle gives off a strong branch, which is attached to 
the under side of the sac and the duct of the ampulla, and then divides, — the end of the 
vas deferens passing between its branches, — becoming attached to the upper and lower 
sides of the sac. A number of irregular fascicles arising from the body-wall were 
attached to the dart-sac, and to the penis (fig. 14, a), and served the purpose of protractors. 
The dart-sac is deeply cleft at the hinder end for about half its length. The right 
half forms a small special sac — th e, penis (figs. 14, h, 15, b), which by a slit-like opening 
communicates with the middle of the cavity of the larger sack (fig. 15). The larger left 
half, which forms the proper sac of the dart (figs. 14, b, 15, d), has a strong, muscular, 
but not specially thickened, wall ; its cavity is nearly filled with the strong dart-cone ; 
through the narrow cavity of this the outer prolongation of the efferent duct, somewhat 
narrower quite at the end, runs towards the slit-shaped orifice at the point ; in the last 
fourth this outer part of the duct was attached to the wall of the dart-cone, otherwise 
it was free and accompanied by one or two nerves, an artery, and two thin muscle-slips. 
The surface of the dart-cone is covered by a simple epithelium ; towards the apex are 
a number of unicellular glands. On the inside of the muscular wall (fig. 18, c) of the 
efferent duct the proper duct of the gland (fig. 18, d) is to be found, with its epithelium 
and thick cuticle, which latter passes in front into the nearly cylindrical, about 4 mm. 
long, brown-yellow coloured strong dart. This dart (fig. 18, a), together with the proper 
duct, can be easily removed from the cavity of the outer duct. It is straight, somewhat 
swollen (fig. 18, b) in the hinder fifth, being here of about '43 mm. diameter, w hil e the 
apex is not generally more than T8 mm. ; the opening at its point lies at the side, and 
is dilated behind into a slit (fig. 20) ; the wall of the dart is rather thin, thicker quite 
posteriorly ; in the wall the branched bone corpuscle-like cells were rather inconspicuous. 
I did not succeed in making out the structure and relations of the foot gland. 
To compare with this species I have also investigated the following species, which is 
closely allied to it. 
Onchidium tonganum, Quoy et Gaimard (PI. YI. fig. 19 ; PI. VII. figs. 1-6). 
Onchidium tonganum, Quoy et Gaimard, Voyage de 1’ Astrolabe, Zool. Moll., t. ii., 1832, p. 210, 
pi. xv. figs. 17, 18. 
,, „ Semper, Reisen im Archip. d. Philipp., Th. II. Bd. iii., Landmollusken, 
Heft v., 1880, pp. 258-260, Taf. xix. figs. 2, 9, Taf. xxii. figs. 1, 2, 10. 
Habitat. — Pacific, Indian Ocean. 
The specimen which I investigated was obtained by Professor Reinhardt during the 
