ON SPLANCHNOTROPHUS. 57 
under consideration, must derive their oxygen from that absorbed by the animals they 
infest. They extract it, most probably, either directly from the blood or from the exuded 
serum, which will at once nourish and aérate the tissues of the parasites, in the same 
manner as it acts upon the tissues of the animals from which they extract it. It is appa- 
rently in consequence of this low condition of the respiratory function that the ovigerous 
sacs of the parasite are placed, as we have seen, in the vicinity of the gills of the Nudi- 
branch, where the ova obtain the advantage of the branchial currents of the infested 
animal. And it is interesting to remark that most of the allied forms of Lerneoidea, in 
which there are no respiratory organs, take up their abode on the gills of the sustaining 
animal, or at least in such a position that the branchial currents must pass over them. The 
Caligoidea, moreover, in which the respiratory organs are of low functional significance, 
avail themselves of the aid thus afforded by the animals they inhabit, some taking up 
their abode on the gills, others near to the fins of various fishes, whilst even those which 
attach themselves to the surface of the body are exposed to the almost perpetual currents 
occasioned by the general movements of the fish through the water. 
In consequence of the opacity and soft condition of the specimen examined, the internal 
- anatomy could not be made out; but, from the perfectly animalized nature of the food, 
we may fairly assume that the alimentary canal is very simple in its structure. We have 
seen that the organs of the mouth are minute and feeble, formed apparently for piercing 
and wounding the tissues, so that the fluids may escape. The fluids are imbibed perhaps 
chiefly by the aid of the largely developed labrum; for, although it is not in the form of 
a suctorial proboscis or tube, yet the posterior angles, which we have described as consi- 
derably produced, may, in life, be still more so, and perhaps entirely encircle the man- 
dibles, so that an incomplete suctorial disk or tube may thus be extemporized. 
Neither has much respecting the reproductive system been satisfactorily determined. 
The ovaries are lobulated organs, and occupy the greater portion of the cavity of the 
thorax, extending into the arm-like lateral processes even to their extremities. The 
ovigerous sacs are appended to the lateral angles of the first abdominal segment, through 
which each communicates with its ovary by a slit-like orifice placed diagonally (Pl. XV. 
fig. 4 b, & Pl. XVI. fig. 3). In one instance the sacs disappeared the second day after 
capture, and in the course of a few hours were replaced by others, in which the ova were 
evidently in a much less advanced condition of development. 
The testes appear to be irregular-formed masses, lying in the thoracic cavity, in contact 
with and on either side of the alimentary canal. There are two elongated pyriform 
seminal vesicles (Pl. XV. fig. 11 d), placed in the posterior thoracic segments, with their 
attenuated extremities directed downwards, and terminating at the sides in two tubercular 
swellings (Pl. XV. fig. 11e, & Pl. XVI. fig. 10) situated at the junction of the genital and 
succeeding abdominal segments. These swellings are the roots of the intromittent 
organs, through which their extremities may be seen occasionally protruding in the form 
of curved processes of no great length. They each communicate with their respective 
seminal vesicle by a short constricted duct-like tube, but the connexion of these vesicles 
with the testes could not be determined. 
There can be no doubt that our two new parasites are rightly associated with the 
VOL. XXIV. r 
