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



Under these circumstances it will not be uninteresting to 

 learn that our quarter of the globe possesses two additional 

 species of the family Argulidse, — one a marine form, from South- 

 ern Europe (the Mediterranean), the other a freshwater form, 

 like A.foliaceus, and belonging to the middle and northern parts 

 of Sweden. The first of these two species, Argulus purpureus, 

 has certainly been already described by Risso under the name of 

 Binoculus bicornutus and Agenor purpureus ; but it seems to have 

 been altogether overlooked by later observers, the reason for which 

 .should no doubt be sought partly in the limited circulation 

 which many of Risso's works have attained, partly in that 

 author's usually very insufficient descriptions, which often render 

 the recognition of the species intended very difficult : Agenor 

 purpureus is referred to the family Bopyridse ! A new descrip- 

 tion of this species, of which I found an example at Nice, on the 

 pectoral fin of Pagellus erythrinus, will, therefore, not be consi- 

 dered superfluous. 



The other, the Scandinavian species, which I call Argulus 

 coregoni, is, on the contrary, new to science. My attention .was 

 drawn to it thi'ough a memoir by Dr. C. L. Nystrom*, wherein 

 mention was made of an unusually large Argulus as one of the 

 parasites which infest the Coregonus in Jemtland. Specimens 

 of this Argulus were brought by Dr. Nystrom to the Royal 

 Zoological Museum in Stockholm, where also specimens exist 

 from Dalsland collected by Mag. H. Widegren ; and through 

 the united kindness of Prof. Loven and Hrr. Nystrom and 

 Widegren, I have had opportunities of examining both Jemt- 

 land and Dalsland specimens of this large and well-marked 

 species. 



I. 



Before proceeding to the description of the animals in ques- 

 tion, it will be advisable to state my own conceptions of the 

 various divisions of the body and its accessory organs, which have 

 been very difi^erently interpreted by the authors who have hither- 

 to handled this group. The first great division of the body, 

 which bears the antennse, the organs of the mouth, and the fol- 

 lowing two minute pairs of limbs, and which in these animals is 

 developed into a large shield produced behind into two lobes, I 

 call the head or head-shield (scutum cephalicum) ; the other, to 

 which the cloven swimming-feet are attached, the trunk (truncus), 

 which is followed by a tail (cauda) transformed into a leaf-like 

 respiratory plate, bearing two small appendages, which, in the 

 newly-hatched larva, are situated at the tip of the tail (as is the 



* " lakttagelser roranrle Faunau i Jemtlands Vattendrag," Akadetnisk 

 Af handling, &c. (186.3), p. 1.9. 



