TBEMATODA. 



325 



form an X-shapcd double animal, the posterior ends of which are provided with 

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

 as Dlporptt ; they then possess a ventral sucker and a dorsal papilla (21)0 a, G 

 and Z~). In the double animals the formation 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. 2(>1, &). which only differs from 

 Dipnri>a 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 oppor- 

 tunity of fixing itself on the gills of a 

 fresh-water fish occurs, the young animal 

 loses its cilia and becomes a Diporpa, 

 which possesses, besides the characteristic 

 apparatus for attachment, the alimentary 

 canal, and the two excretory canals with 

 their openings at the anterior part of the 

 body (at the level of the pharynx), and 

 sucks the branchial blood. The junction 

 of the two Diporpa soon follows ; and 

 this does not take place, 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. 260. V). D. parailo.ritm 

 v. Nordm.. on the gills of many fresh- 

 water fish. 



Fam. Gyrodactylidae. Very small Tre- 

 matodes with large terminal caudal disc 

 and powerful hooks. They are viviparous, 

 producing a single young one (first gene- 

 ration) at a time, within which, while 

 still in the body of the parent, another 

 young one (second generation) may be 

 present, and in this yet another (third 

 generation). V. Siebold believed that he 

 had observed a young animal developing 

 from a germ cell of Gyrodtu-t i/ln.*. and 

 that this became pregnant during its 

 development. He regarded the (T //>/>- 

 dactyliis as an asexual form, since he 

 failed to find organs for the production 

 of sperm. G. Wagener, however, showed 

 that the reproduction is sexual, and 

 conceived the idea that the germs from which the second and third generations 

 are formed are derived from the remains of the fertilized ovum from which 

 the first generation is formed. Metschnikoff, too, is of the opinion that the 

 individuals of the first and second generations are formed at the same time 

 from a common mass of similar embryonic cells. Gyrodnctyltix v. Xordm., 

 ff. clegans v. Xordm., from the gills of Gyprinoids and fresh-water fish. 



FIG. 26:.'. Tienia Kii/jinntn (mediocanellata), 

 natural size (after R. Leuckart). 



