M. T. Thorell on the Systematic Position of the Argulide. 275 
paratus, he says that Argulus makes a nearer approach to the 
Ostracoda “in its thread-like zoosperms and the existence of 
male accessory glands and of female seminal receptacles.” He 
might have added that they here also approach the Copepoda, 
which are also sometimes furnished with thread-like zoosperms 
(I myself have observed them in Lichomolgus) ; accessory glands 
are often met with in connexion with the male organs, and a 
single or double receptaculum seminis apparently always, in the 
Copepoda. Resemblances in these particulars, with the excep- 
tion of the last named, are, however, relatively of little weight. 
Meanwhile the generative apparatus in the Argulide is referable, 
as we have seen, in certain points to the Branchiopods, in others 
to the Copepods. But conditions in the highest degree peculiar 
separate them from both these orders. Unfortunately, much 
respecting the propagation of the Argulidz remains still obscure; 
and even Leydig’s accurate researches on this subject* leave many 
important questions unanswered. The signification of the 
characteristic (probably copulatory) appendages on the two or 
three posterior pairs of swimming-feet is undetermined, with 
the exception of the capsule on the hinder part of the third pair 
of feet, which, according to Leydig, is filled with sperm from 
the opening of the ductus deferens, and afterwards, during the 
act of copulation, presses its contents into the seminal reservoirs. 
The function which belongs to the “hook” on the last pair of 
legs, which in its structure so strongly recalls the copulatory 
organ of the Spiders and Ju/us, is unknown. Leydig perceived 
only that during copulation this was “closely pressed upon or 
into the capsule of the last pair of legs but one,” and that it did 
not serve in any way to retain the female. This latter function 
may perhaps belong to the projections on the hinder part of the 
second pair of legs, which occur in certain species, as A. coregont. 
As to the significance of the projections on the fore part of the 
third pair of feet, I do not venture to guess. 
The females of the Argulidze have, as has been said, two recep- 
tacula seminis, which, each by means of a proper channel of 
communication, opens out on a moveable papilla near the under- 
side of the tail, behind the mouth of the ovarium; and thus it 
would seem that an immediate connexion between the receptacles 
and the ovarium is here wanting. How the semen under these 
conditions can come into contact with the eggs is still undis- 
covered. 
Another circumstance also brought to light by Leydig (with 
reference to A. foliaceus) is of the greatest interest. ‘ The 
receptacle is,” he says, “in females which have never copulated, 
empty and folded inwardly. After copulation there appears in- 
* Ueber Argulus foliaceus, p. 339 &e. 
