154 M. T. Thorell on the Morphology of the Argulidse. 



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 antennae 

 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 Caligidse (the first 

 pair of maxillse, according to M. -Edwards and others), which 

 Kroyer designates " antennpalp," but is called " accessory hook 

 of the second pair of antenuse" (" 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 Caligidse and theArgulidse. With 

 respect to the latter, this is indeed evident from the circumstance 

 that the Argulidse 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 antennse 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 Argulidse, as far as these are known to us. Former 

 authors who have occupied themselves with researches concern- 



* " Bidrag til Kundskab cm det aabne Havs Snyltekrebs og Leraaeer, 

 &c.," Kongl. Danske Vidensk. Selsk. Skrift. 5te Raekke, Bd. v. (1861) 

 p. 350. 



