STRUCTURE OF TAPE -WORMS. 153 



ers and two large median ventral hooks on the hinder end. 

 The ripe eggs are deposited in the water in winter, when 

 the ciliated young, with four eyes and without suckers, find 

 their way into the gill-cavities of tadpoles, whence, during 

 or after metamorphosis, they pass into the urinary bladder 

 of young frogs ; P. integerrimum Eudolphi lives in that 

 of Rana temporaria (Clans' Zoologie). 



A case of budding or parthenogenesis is said to occur in 

 the genus Gyrodactylus. This is a very small Trematode 

 with a large terminal disk, bearing a peripheral set of pow- 

 erful hooks, with two long curved median spines. The 

 body of the hermaphrodite worm shelters a daughter, a 

 granddaughter, and great-granddaughter generation. G. ele- 

 gans Nordmann lives on the gills of Cyprinoid and other 

 fresh-water fishes. Dactylogyrus lays eggs, not being par- 

 thenogenetic ; it has four head-flaps. D. amphibothrium 

 Wagener lives on the gills of the stone -perch ; D. fallax 

 Wagener on Cyprinus rutilus. 



Order 3. Cestodes. The common tape-worm is the type 

 of this order. Specimens may be procured from physicians, 

 and a careful examination of cross-sections and ordinary 

 dissections will convince the student that the tape-worm has 

 no mouth, although a head armed with suckers or hooks. 

 The body is divided into an enormous number of segments 

 or proglottids, but there is no digestive canal, the worm 

 living immersed in the contents of the intestines of its host ; 

 its food being absorbed from the juices of its host through 

 the walls of the body. 



The tape-worms and their allies are not known with cer- 

 tainty to have any trace of a nervous system. The water- 

 vascular system is well developed in the Cestodes, where it 

 seems to be excretory in its functions, as in the Trematodes. 

 There are usually four, sometimes only two, longitudinal 

 canals, which are connected in the head and in each segment 

 with transverse anastomosing branches, while from these main 

 canals a network of fine vessels branch out. Granules and 

 whitish chalky deposits occur in the canals, and these con- 

 cretions, like similar bodies in the excretory canals of Tre- 

 matodes, seem to have, Leuckart claims, a relation like that 



