ANNULOIDA I SCOLECIDA. I 89 



pisiformis) ; and the tape-worm of the dog (Ttznia serrata) is 

 the developed form of the Ccenurus cerebralis of the sheep, the 

 cystic worm which causes the " staggers " in the latter animal 

 Besides tape-worms, however, man is liable to be affected 

 with " scolices," which are the larvae of the tape-worms of other 

 animals. Thus, what are professionally called " hydatids " in 

 the human subject, are really the scolices of the tape-worm of 

 the dog. The disease is indicated by the presence of the so- 

 called " hydatid-tumour," which consists of a strong membran- 

 ous cyst the " hydatid " proper situated in some solid organ, 

 most commonly the liver, and filled with a watery fluid. To 

 the interior of the cyst are attached numerous minute scolices, 

 many others also floating freely in the contained fluid. These 

 " Echinococci," as they are called, do not differ in structure 

 from other scolices, consisting of a head, provided with four 

 suckers and a circlet of recurved hooklets, a vesicular body, 

 and an intermediate contracted portion or neck. The Echi- 

 nococci multiply within the hydatid cyst by gemmation, but 

 they develop no reproductive organs. If, however, an Echi- 

 nococcus should gain access to the alimentary canal of a dog, it 

 then becomes the tape-worm peculiar to that animal the Tcenia 

 echinococcus. 



CHAPTER XXV. 

 TREMATODA AND TURBELLARIA. 



ORDER TREMATODA. This order includes a group of animals, 

 which, like the preceding, are parasitic, and are commonly 

 known as " suctorial worms," or " Flukes." They inhabit vari- 

 ous situations in different animals mostly in birds and fishes 

 and they are usually flattened or roundish in shape. The 

 body is provided with one or more suctorial pores for adhesion. 

 An intestinal canal, with one exception, is always present, but 

 this is simply hollowed out of the substance of the body, and 

 does not lie in a free space, or " perivisceral cavity." The 

 intestinal canal is often much branched, and possesses but a 

 single external opening, which serves alike as an oral and an 

 anal aperture, and is usually placed at the bottom of an an- 

 terior suctorial disc. The sexes are united in the same indi- 

 vidual. A " water-vascular system " is always present, and is 

 sometimes " divided into two portions, one with contractile and 

 non-ciliated walls, the other with non-contractile and ciliated 

 walls " (Huxley). 



