M. T. Thorell-on the Morphology of the Argulide. 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 Caligide: the append- 
ages of the Argulide should, according to him, represent not 
only the appendages, but the whole of the tail behind the genital 
ring in the Caligide—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 Caligide and many other (especially the pceci- 
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 Coryceus; and just 
such, in fact, is the stage of development of the tail in Argulus. 
Now, if the unsegmented tail of Coryceus corresponds to the 
tail inclusive of the genital ring in the Caligide, which no one 
presumes to doubt, so also must the tail in the Argulide corre- 
spond to the entire tail in the Copepoda and Caligidz in general. 
Still less correct than Kroyer’s is Gegenbaur’s view of the 
hindmost segment of the body: Gegenbaur+ regards it as 
consisting of “a pair of partly coalesced branchiz,” and takes it as 
corresponding not only physiologically, which would have been 
perfectly correct, but even morphologically with the branchiz 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{. 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 larvee, 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 Argulide have been thus 
* Loc. cit. p.88. 
+ Grundziige der vergleichenden Anatomie (1859), pp. 245-246. 
t Vide Leydig, ‘‘ Ueber Argulus foliaceus, ein Beitrag zur Anatomie, His- 
tologie und Entwicklungsgeschichte dieses Thieres,” Zeitschrift fiir wis- 
senschaftliche Zoologie, Bd. 11. (1850) pp. 338-339. 
