154 M.T. Thorell on the Morphology of the Argulide. 
itself of these “for cleaning the sucking-cups and for removing 
extraneous particles from the cavity.” Their first joint usually 
shows on the underside a plain, sharply-defined surface, which, 
posteriorly, is mostly produced into three strong teeth, and 
which we therefore, in common with Kroyer, call the comb 
(“kammen,” pecten). 
Immediately behind the base of the second pair of antennze 
is a strong chitinous tooth or hook, which is regarded by Kroyer 
as a palp. In position and external appearance it agrees, indeed, 
with the hook, frequently occurring in the Caligidze (the first 
pair of maxillz, according to M.-Edwards and others), which 
Kroyer designates ‘“ antennpalp,” but is called ‘“ accessory hook 
of the second pair of antenne” (“hjelpkrok,” hamulus) by 
Steenstrup and Liitken*, and is not regarded, therefore, by 
them as belonging to the true series of appendages, but as a 
cuticular growth appertaining to the dermal framework. This 
view seems to be shared also by Claus, and is, in my opinion, the 
correct one, both as regards the Caligide and theArgulide. With 
respect to the latter, this is indeed evident from the circumstance 
that the Argulide often possess two extra pairs of perfectly simi- 
lar hooks,—the one between the second pair of footjaws, the other 
behind these, before the base of the first pair of swimming-feet, 
which hooks cannot easily be looked upon as reduced appendages 
or parts of such. On the basal joint of both the first and second 
pairs of antennz and the second pair of footjaws the chitinous 
covering is also‘developed into similar hooks ; and certain parts of 
the underside of the body, especially the border of the head- 
shield anteriorly, are usually thickly set with small teeth, which 
have a similar significance with the larger hooks previously men- 
tioned, and, like them, serve to fix the creature on the spot to 
which it has attached itself by suction or clinging. All these 
hooks and teeth have, accordingly, the apex directed backwards, 
and thus prevent the animal from sliding or being brushed from 
before backwards, the only direction in which any strong pres- 
sure under ordinary conditions could operate. This modifica- 
tion of cuticular structure should consequently be stronger in those 
species which principally subsist on the outer integument of 
fishes, as, for instance, A. foliaceus, coregoni, purpureus, and 
weaker in such as subsist in the gill-cavity, like A. catostomi. 
It remains for us briefly to account for the true mouth-organs 
in the Argulide, as far as these are known to us. Former 
authors who have occupied themselves with researches concern- 
* « Bidrag til Kundskab om det aabne Havs Snyltekrebs og Lerneer, 
&e.,”’ Kongl. Danske Vidensk. Selsk. Skrift. 5te Reekke, Bd. v. (1861) 
p. 350. 
