164 MANUAL OF ZOOLOGY. 



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.) 



The Trematode Worms are all hermaphrodite, and they pass 

 through a series of changes in their development somewhat 

 analogous to those observed in the Tceniada. This subject, 

 however, is still involved in great obscurity, and it is too com- 

 plicated to admit of description in this place. The larvae are 

 often tailed, but never possess cephalic hooklets, and are never 

 " cystic." 



From the absence of a perivisceral cavity, the Trematoda 



