DISTOMA HEPATICUM. 141 



larvae a kind of pedicle exists, by which they are attached to the walls 

 of the cyst. In a quite recent state the larvae have been seen swimming 

 actively in the liquid of the cyst by means of cilia upon the surface of 

 the body. They appear usually to be developed from the interior of the 

 cyst ; but, as Kuhn long since showed, they are sometimes produced by 

 external gemmation : the contents produce a slight protrusion of a part 

 of the wall of the cyst ; the protruded portion enlarges, afterwards 

 becoming constricted at the base, and at last probably separates from the 

 parent, to become itself a parent in a similar manner. Hence it appears 

 that the larvae cannot be regarded as the parasites of the cyst, but must 

 be viewed as arising from a partial segmentation of the contents of the 

 parent. The Echinococci do not acquire their full development into 

 Tcenice unless they reach the alimentary canal. The cysts and their 

 contents, including the Echinococci, undergo a kind of degeneration, 

 becoming partially converted into fatty or calcareous matter, or the 

 entire contents become amorphous and granular, the hooks remaining 

 longest unaltered, but finally disappearing also*. 



(369.) TKEMATODA. In the fluke, Distoma (Fasciola, Linn.) hepati- 

 cum, we have an Entozoon of more complex and perfect structure one 

 of those forms, continually met with, which make the transition from 

 one class of animals to another so insensible that the naturalist hesi- 

 tates with which to associate it. 



(370.) The Distoma is commonly found in the liver and biliary ducts 

 of sheep and other ruminants, deriving nourishment from the fluids in 

 which it is immersed. The body of the creature, which is not quite 

 an inch in length, is flattened, and resembles in some degree a minute 

 sole or flat-fish. At its anterior extremity is a circular sucker or disk of 

 attachment, by which it fastens itself to the walls of the cavity in which 

 it dwells, as well as by means of a second sucker of similar form, placed 

 upon the ventral surface of the body. In the annexed diagram (fig. 71) 

 the posterior sucker has been removed in order more distinctly to exhibit 

 the internal structure of the animal. The name which this Entozoon 

 bears seems to have been given to it from a supposition that it possessed 

 two mouths, one in each sucker, whereas the anterior or terminal disk 

 (a) only is perforated, the other being merely an instrument of adhe- 

 sion. The alimentary canal (6) takes its origin from the mouth as a 

 single tube, but soon divides into two large branches, from which rami- 

 fications arise that are dispersed through the body, each terminating in 

 a blind clavate extremity. These tubes, from being generally filled 

 with dark bilious matter, are readily traced, even without preparation, 

 or they may be injected with mercury introduced through the mouth. 



* Kuhn, Ann. Sc. Nat. 1 ser. xxix. p. 273 ; Siebold, Wiegmann's Archiy, 1845, and 

 Siebold and Kdlliker's Zeitschr. iv. ; G-luge, Ann. Sc. Nat. 2 ser. viii. p. 314; Owen, 

 Hunterian Lectures, i. p. 46 ; Dujardin, Helminthes, p. 635 ; Huxley, Ann. Nat. Hist. 

 2 ser. xiv. p. 379. 



