On a New Ichthyoxenus (/. opisthopterygium sp. nov.) 



from Lake Biwa. 



By 



Shigemi Ishii, Rigakusfd. 



In the abdominal cavity of AcJicilognathus tabira Jordan & Thompson, 

 • collected at Hikone (Lake Biwa) last summer, I have found an apparently- 

 new Ichthyoxenus species which differs markedly from Ichthyoxenus 

 japonensis Richardson. 1 ' The new parasite will be called Ichthyoxenus 

 opisthopterygium. It seems that the occurrence of the parasite is not 

 unfrequent, as I have found ten cases of its occurrence out of sixty-nine 

 individuals examined of the host-fish. In each case the parasite was 

 present in a single number, thus giving me in all ten specimens of it, of 

 which number three were males and seven females. 



The manner of occurrence of /. opistlioptcrygiuvi in the host body 

 seems to be the same as that of /. japonensis, being found inclosed in a 

 membranous sac which lies freely in the body-cavity and opens by an 

 orifice situated close behind the pectoral fin of either side. Inasmuch as 

 the tissue of the sac apparently belongs to the host and the parasite does 

 not stand in direct contact with the latter's viscera, it is, strictly speaking, 

 improper to say that the parasite is lodged in the visceral cavity itself. 



The females lie in the sac usually with their head directed towards 

 the postero-lateral side and their back turned towards the ventral side of 

 the host-body. The males, on the other hand, take a somewhat vertical 

 position, with their head directed more or less towards the dorsal side of 

 the host. In both sexes, the tail-end is placed always near the orifice of 



i) Harriet Richardson', " Ths Isopod genus Ichthyoxenus Herklots, with description of a 

 species from Japan,'' Proc U. S. Nat. Mus., Vol, 45, June, 1913. 



