HETEROCOTYLEA. 



235 



organ at the hind end, hearing suckers and hooks; usually with two oral suckers. 



On the gills of fishes, in Amphibia and Reptiles on the skin, nasal passages, or 



bladder. 



Sub-fam. 1. Octocotylida . With two oral suckers and genital hooks ; 

 adhesive organ with 4, usually 8 small suckers. On the gills of marine and 

 f.w. fish. Octobothrinm Leuck. ; Plcurocotyle Gerv. and Ben.; Diplo~oon 

 v. Nordm. (Fig. 187). The animal is double, two individuals being fused 

 to form an x-shaped double animal, the posterior ends of which are provided 



Fio. 187. Young Diplozoon (after Zeller). , two young Diporpae beginning to attach them- 

 selves together ; b, after complete attachment. mouth ; H posterior suckers ; Z dorsal 

 papilla ; G ventral sucker. 



with two large suckers divided into four pits. In the young state they 

 live solitarily as Diporpa ; they then possess a ventral sucker and a dorsal 

 papilla (Fig. 187, ). The Diporpa is without generative organs ; these are 

 not formed until after it has fused with another Diporpa, to form the 

 x-shaped Diplozoon (Fig. 187, b). The production of ova is confined to 

 a definite period of the year, 

 usually the spring. The eggs 

 are laid singly after the forma- 

 tion of the thread by which 

 they are attached, and two 

 weeks later the embryo (Fig. 

 188), which only differs from 

 Diporpa in the possession of 

 two eye - spots and a ciliated 

 apparatus upon the sides and 

 on the posterior extremity of 

 the body, is hatched. When 

 an opportunity of fixing itself 

 on the gills of a fresh -water 

 fish occurs, the young animal 

 loses its cilia and becomes a Diporpa, which possesses the characteristic 

 apparatus for attachment, and sucks the branchial blood. The junc- 

 tion of two Diporpae can now take place ; this is not effected as was 

 formerly believed, by the fusion of the two ventral suckers, but in such 

 a manner that the ventral sucker of each animal affixes itself to the dorsal 

 papilla of the other, and fuses with it (Fig. 187, b). In this fusion the 

 vas deferens of the one animal is directly connected with the opening of 

 the vagina of the other. D. paradoxum v. Nord., on the gills of many 

 fresh-water fish. Antlwcotyle Ben. and Hesse; Vallisia Perugia-Parona ; 



PIG. 188. Egg () and larva (b) of Diplozoon 

 (after Zeller). 



