REPORT OjST THE NIJDIBRANCHIATA. 
105 
pale yellowish cuticle, which forms strong folds with here and there scattered disk-like 
structures. These disks are arranged in irregular (quincuncial) series, of which there 
are from 7 to 8 below and from 4 to 5 above; altogether, I found seventy-two of these disks. 
They are of roundish or oval contour, and both forms occur above (PI. III. fig. 23) and 
below (PI. II. fig. 9) within the prseputium. Their greatest diameter varies between 
•8 and - 25 mm., and plates of the two extreme sizes were found both above and below. 
The more or less arched upper surface (fig. 8) of the disks is prolonged into a hook, 
about T8 mm. high, which is often very sharply bent ; the stroma of the disk is 
frequently prolonged into the hook (PI. II. fig. 24), in others the stroma seems to be 
withdrawn, leaving small air-containing spaces (fig. 24). At the base of the cavity of the 
prseputium is the cylindrical glans, about '8 mm. long, and covered with a simple cuticle ; 
it is traversed by the seminal duct, opening by an oval aperture at its summit (fig. 23). 
As it passes into the vestibulum the prseputium is strongly pigmented. At the lower end 
of the penis opens the whitish vestibular gland (PI. III. fig. 12, Jc), about the size of the 
prostate, and rather compressed ; it is sessile and has a largish cavity. The spermatheca 
(PL II. fig. 22, a) is spherical and about 6 mm. diameter, whitish in colour, and filled with 
semen and detritus. The stout vaginal duct is, including the long vagina (fig. 22, b), about 
12 mm. long, and lined throughout by a strong yellow cuticle raised into 6-8 folds, which 
are especially strong in the vagina, and can be seen from the outside ; the lowest end of 
the vagina (fig. 22,c) is brown coloured. The uterine duct (fig. 22, dd) is thinner and shorter 
than the vaginal, only 6 mm. long ; at its lower end is the spermatocyst (fig. 22, e) attached 
by a short stalk ; it is sac-shapecl, yellow in colour, about 3*5 mm. in length, and was full 
of semen. The broad white upper margin of the mucous gland has fine gyri ; quite 
in front there is a more yellowish portion ; the rest of the outer side of the mucous 
gland is 'white, with a few thick windings. The larger part of the hinder side and the 
under margin was taken up by the large dirty yolk-yellowish albuminiparous gland; 
the duct of the mucous gland has the usual internal fold ; the vestibulum genitale shows 
logitudinal folds. 
In all probability the form described here is identical with the Platydoris eury- 
chlamys, formerly investigated by me, though it presents a few differences, such as the 
greater number of disks in the prseputium and their somewhat different form. An 
investigation of a number of individuals, however, would perhaps show a great variability 
in the armature of the penis ; and perhaps the species Platydoris vicina, lately instituted 
by me, 1 may prove to be merely a variety of this. 
Another specimen of the same species was dredged during the Challenger Expedition 
on the Eeef of Tongatabu, from a depth of 8 fathoms. It was strongly bent ; when 
1 Supplementheft i., 1880, pp. 62, 63, Taf. E. figs. 16-20. 
(ZOOL. CHALL. EXP. — PART XXVI, — 1884.) 
Cc 14 
