M. T. Thorell on the Morphology of the Argulidae. 151 



case in the allied forms Phyllopoda and Copepoda), but, in the 

 adult animal, have gradually advanced upwards to the base of 

 the incision which divides the respiratory plate into two lobes. 

 The trunk is named by some authors thorax, by others abdo- 

 men, according as they call the first division of the body head 

 (cephalic shield) or cephalothorax ; the last segment is, in a 

 similar manner, regarded either as the abdomen or postabdomen, 

 in descriptive works mostly called cauda. Kroyer* regards it as 

 the genital ring, in consequence of his having, curiously enough, 

 considered it to correspond only to the so-denominated foremost 

 segment of the " postabdomen^' in the Caligidse : the append- 

 ages of the Argulidse should, according to him, represent not 

 only the appendages, but the whole of the tail behind the genital 

 ring in the Caligidse — and hence, naturally, in all the other 

 Copepoda. But the genital ring is nothing but the coalesced 

 first two segments (or only the first segment) of the tail, which 

 in the Copepoda is usually set apart for the functions of genera- 

 tion, and in the Caligidse and many other (especially the poeci- 

 lostome and siphonostome) Copepoda attains a greater develop- 

 ment, especially in breadth, than the following caudal segments. 

 The number of these varies much, being four or less : omsetimes, 

 even, the tail remains unsegmented and consists of a single piece, 

 for instance, in some species of the genus Cory cans; and just 

 such, in fact, is the stage of development of the tail in Argulus. 

 Now, if the unsegmented tail of Corycceus corresponds to the 

 tail inclusive of the genital ring in the Caligidse, which no one 

 presumes to doubt, so also must the tail in the Argulidse corre- 

 spond to the entire tail in the Copepoda and Caligidse in general. 

 Still less correct than Kroyer's is Gegenbaur's view of the 

 hindmost segment of the body: Gegenbaurf regards it as 

 consisting of " a pair of partly coalesced branchise,'' and takes it as 

 corresponding not only physiologically, which would have been 

 perfectly correct, but even morphologically with the branchise of 

 the Crustacea. Gegenbaur's assertion that this view is shared by 

 Leydig would seem to be the result of a misunderstanding of that 

 author's meaning J. In the larval state the tail in Argulus has 

 a form which easily shows the incorrectness of Gegenbaur's 

 view : it is then exactly like the tail in the older Phyllopod and 

 Copepod larvse, and bears, as already mentioned, the usual ap- 

 pendages at the tip, between which the anal opening is situated. 

 If the various body-segments of the Argulidse have been thus 



* Loe. cit. p. 88. 



t Grundzuge der vergleichenden Anatomic (1859), pp. 245-246. 



I Vide Leydig, " Ueber Argulus foliaceus, ein Beitrag zur Anatomic, His- 

 tologic imd Entwicklungsgeschichte dieses Thieres," Zeitschrift fiir wis- 

 Benscbaftliche Zoologie, Bd. ii. (1850) pp. 338-339. 



