42 
In Homoeoscelis minuta (pl. I, fig. 1i and fig. 1k) I have found the rather small spermato- 
thecee (though they are not illustrated) close together in the line between the hindmost pair 
of trunk-legs, and I think I have found the two genital apertures in close proximity in the 
posterior wall of the ventral depression — which is particularly conspicuous in fig. 1k — 
in a line between the first pair of legs. 
In a number of species belonging to Spheronella I have found a most peculiar struc- 
ture beneath the larger part of the skin of the head at its back and sides. It appeared most 
distinctly in S. paradoxa, where I saw very plainly beneath the skin a single layer of 
rather large hollow spaces; fig. 4k in pl. IIT is drawn to the same scale of enlargement 
as fig. 4i and shows the skin and two rows of the afore-mentioned hollow spaces beside 
each other. In S. Metope (pl. IV, fig. 3i) the spaces were filled and appeared in outline 
as shown in the illustration. 
The males are sometimes hinged on the females, but much more frequently on the 
gills or on the marsupial plates of the host by a thread which proceeds from the median 
line of the front close in advance of the rostrum. This thread is secreted by a gland or 
glands and can presumably be produced by the males of all species. The shortest thread I 
found in Homocoscelis minuta, in two specimens, in one of which its length was similar to 
that of the first joint of the maxilliped, in the other somewhat shorter. In e. g. Spher. para- 
doxa (pl. IL, fig. 4h,s) the thread is about 3/s of the length of the animal, in Stenothocheres 
Sarsvi (pl. I, fig. 2k,s) a little shorter and in S. abyssi (pl. IV, fig. 2 d) even a little longer 
than the whole animal. I found the longest thread in a specimen of Aspidoecia Normani, 
where it was between twice and three times as long as the animal, whereas in the spe- 
cimen illustrated in pl. XII, fig. 3k it was scarcely half as long as the male. This last 
instance shows that the length of the thread can vary very much in the same species, but 
this is not usually the case, as in some species a shorter, in others a long thread is always 
found. In all the above-mentioned and in several other species the thread is always simple 
and cylindrical, generally a little dilated towards the distal end by which it attaches itself, 
and not unfrequently the end is expanded into a disk. Deviating forms of this thread are 
met with in the species of the genus Mysidion, and especially in the species which I have 
placed together below under the heading of the Spheronella Leuckartii-group. In Mysidion 
the proximal part of the thread is simple, the distal part appears in two varieties; either, 
as in Mysidion abyssorum (pl. XII, fig. 2g), it shows two considerable fusiform expansions, 
the middle parts of which are each surrounded by a peculiar collar-shaped ring, or, as in 
Mysidion commune (pl. XI, fig. 3h,s), the apical part is very thick and above it the 
thread dilates still more and becomes fusiform; its widest part has a collar-like ring, and 
a similar ring surrounds it somewhat higher up, where the thread is only half as wide. In 
the species belonging to the Spher. Leuckartii-group we often find the male hinged by a thread 
Which varies in extent between nearly half and almost the full length of the animal, and is 
constructed in the following way: it is divided into two parts, either of equal length, or 
