194 BKITISH PARASITIC COPEl'OUA. 



Antennae and mouth-appendages somewhat similar 

 to those of Lernaeopoda salmonea. The second maxil- 

 lipeds nearly as long as the genital segment, free 

 except at the tips where they are united to a small 

 obconical chitinous plug. 



We have not seen the male of Achtheres percarum, 

 but it has been described by JSTordmann. It is very 

 small, measuring little more than a millimetre in 

 length. 



Habitat. — Parasitic on the gills of fresh-water perch 

 and trout. On the gills of the common trout, Moray 

 Firth district (T. Edward, in 'Museum Normanianum'). 

 On the gills of a trout captured by Mr. W. S. Caine 

 in Loch Awe, a small fresh-water loch in Assynt, 

 Sutherlandshire (T. Scott) .* On the gills of a trout 

 captured in Loch Tay, Perthshire, by Dr. Williamson, 

 to whom we are indebted for the specimen. 



The specimens recorded above agree generally with the 

 form described and figured by Nordmami, except that the 

 genital segment is not segmented as his drawing shows it to 

 be (this drawing is reproduced on PI. L, fig. 6, for com- 

 parison with Scottish specimens). 



The chitinous plug by which the female Achtheres is fixed to 

 its host is considerably and regularly expanded in front, as 

 shown by fig. 7 ', PI. LIX. This form of plug appears to be 

 characteristic of the genus, and differs entirely from that of 

 the closely allied Lemseopoda salmonea, which is of a broadly 

 Ungulate form. 



Dr. Pehr G-add, in his account of the Copepod parasites 

 found on the fishes of Finland, describes a species parasitic 

 on the fresh-water fish Lucioperca sandra, under the name of 

 Achtheres sandrse.f In that species the genital segment is 

 not segmented, resembling, in that respect, the specimens 

 described above. 



* " The Invertebrate Fauna of the Inland Waters of Scotland," ' Twelfth 

 Annual Report of the Fishery Board for Scotland/ Part III, p. 287 (1894). 



f " Parasit-Copepoder I Finland," ' Acta Societatis pro fauna et flora 

 Fennica/ vol. xxvi, No. 8, p. 22, pi. i, fig. 1 (1904). 



