M. T. Thorell on the Systematic Position of the Argulidse. 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 famished 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 Argulidse 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 Argulidae 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 Julus, 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. coregoni. 

 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 Argulidse 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 &c. 



