middle hooks. Gallien indicates that for D. luscae the 3rd pair is the 

 largest; we did not observe this in D. denticulata. The sizes of the edge 

 hooks in D. luscae are about 0. 025 mm and among D. denticulatua some- 

 what smaller- -0. 02 - 0,023 nrun. The nniddle hooks have a somewhat 

 different shape than the edge hooks: the transversal offshoot is absent 

 at the place of transition of the terminal hook into the handle. The length 

 of the middle hooks of the larvae of ^ luscae is 0. 028 mm and of D. denticulata- 

 0. 03 mm. 



25. The development of Discocotyle Die sing 



The genus Discocotyle (Fig. 228) is distinguished from the 

 closest species by the peculiarities of the structure of the sex system, 

 particularly by the absence of the sex sucker and by the presence of 

 vaginal ducts and follicular testes. Four pairs of clamps of the same 

 size and of the usual type are characteristic for it. We obtained larvae 

 of D. sagittata (Leuckart) fronn individuals taken from the gills of the 

 Whltefish, Coregonus lavaretus (L. ) from the live fish bank (aquarium, 

 nobis) in Leningrad (in October 1951). The free-swimming larva 

 emerged on the 19th day after the deposition of eggs. Unfortunately the 

 larvae came out at night and we could only observe them after they had 

 lost the ciliary epitheliuin. The larvae (Fig. 229) at our disposal already 

 had an unfolded and relatively very large disc and a body flattened dorso- 

 ventrally. The sizes of the larva: general length about 0.42 mm, the 

 width of the body 0. 16 mm; the sizes of the disc 0. 15 x 0. 23 mm. The 

 larva is equipped with 2 pigmented eyes located directly above the 

 rounded pharynx. We did not succeed in examining the internal organi- 

 zation of the larva. The attaching disc of the larva is equipped with 2 p. 201 

 large hooks and 3 pairs of edge hooks and one pair of well-developed 

 attaching clamps (Fig. 230). The large pair of the hooks have a sickle- 

 shaped curved basic part with a rather powerful point. A very long thin 

 handle emerges from the interior edge of its upper end. The length of 

 the entire hook is 0. 082 mm and of the handle 0. 06 mm. In contrast to 

 the middle hooks of the dactylogyrid-type these hooks bend easily at the 

 place of the junction of the handle and the basic part, besides that the 

 handle itself is very resilient and can easily bend. The edge hooks are 

 of the usual type with a strongly developed point; their length is about 

 0. 025 mm. The clamps of the larva have the same structure as in the 

 adult. Their sizes are 0. 06 x 0.008 mmi; thus if we take into consider- 

 ation that among the adult individuals the first pair of clamps has the 

 size of 0. 2 mm x 0. 28 mm we see that in spite of their constant shape 

 the clamps of the larva are many times smaller, consequently they grow 

 powerfully. Data on further development are absent but the general 

 process of the development of the chitinous armature of the disc is clear. 

 It is interesting to note that among adult individuals the eyes are absent, 

 having been reduced during the time of development. 



218 



