BY T. HARVEY JOHNSTON AND 0. W. TIEGS. 91 



■with or without ca«ca. Eyes may be present or absent. Both testis (always 

 single) and ovai-y are unbranehed organs which may be situated laterally or in 

 the midline. The vesieula seminalis may undergo enormous distension. The 

 chitinous penis may be simple or very complex. The vagina may be present or 

 absent, single or double. The vitellaria are never in the form of elongated con- 

 , verging "yolk-tubes" and there is never any connection with the intestine. 



Key to suhfamilies of Gyrodactylidae. 



A. a. Adhesive disc provided with numerous small suckers Merizocotylinae 



b. Disc devoid of such suckers B. 



B. a. Disc provided with a pair (dorsal and ventral) of accessory structures, 



consisting of concentrically arranged rows of scales or broader lamellae . . . . 



LepidotreTthinae 

 b. Disc devoid of such structures C. 



C. a. Disc with four large hooks Tetraonchinae 



h. Disc with two large hooks Gyrodactylinae 



Subfamily I. GYRODACTYLINAE Monticelli, 1892— emend. J. & T. 



Gyrodactylidae in which the disc is distinctly marked oif from the rest of 

 the body, and bears two large hooks, with fourteen or sixteen minor hooks. Head 

 with very distinct lobes related in position to the head-organs. Eyes present or 

 absent. A prominent pharynx. The intestine bilobed and devoid of caeca; a 

 posterior communication between the limbs present or absent. Ovary and testis 

 unbranehed organs. The penis a fairly simple chitinous tube. Accessory copu- 

 latoi-y sti-uetures may be present. A vag'ina present or absent. 



Found on the gills of freshwater and marine fishes. 



3. Gyeodactylus Nordmann, 1832. 



Gyrodactylinae in which the disc bears two lai-ge and sixteen minor hooks. 

 A single paii' of head-lobes present ; no eyes ; intestinal limbs end blindly. Vagina 

 absent; the wona may be vi\'iparous. Gonads situated just behind the middle 

 of the body. 



Found on the gills of freshwater fishes. 



Type, G. elegans Nordm., 1832., from Cyprinus and many other European 

 freshwater fish. 



The following species belong to Gyrodactylus: — G. elegans Nordm., 1832; 

 G. medius Kathariner, 1894; G. ranis Wegener, 1910; G. groenlandicus Levia, 

 1881; G. fairporti van Cleave, 1921. G. gracilis Kathariner, 1894, is perhaps a 

 synonym of G. elegans. 



No membere of the genus have yet been described from Australia. 



4. Dactylogtrus Diesing, 1850. 



Gyrodactylinae in which the disc bears two large and fourteen minor hooks, 

 but the large hooks may undergo considerable diminution in size in some species. 

 Four head-lobes. Intestinal limbs (usually ?) communicate beiiind. Penis 

 generally with an accessory copulatory stnieture. Vagina probably always pre- 

 sent. Gonads situated in, or just behind, the middle of the body. 



Found on the gills of freshwater and marine fishes. 



Type, D. auriculatus (Nordm., 1832) Dies., 1850. 



The genus has not yet been recorded from Australia; Goto and Kikuchi 

 (1917) have, howevei-, described a form, imder the name D. inversus ^ from 

 Japan. 



