ANNULOIDA: SCOLECIDA. 147 



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. ATI 

 intestinal canal 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 anterior suctorial disc. The sexes 

 are united in the same individual. A ' water-vascular system ' 

 is always present, and is sometimes ' divided into two por- 

 tions, 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 booklets and are never 

 ' cystic.' 



Erom the absence of a perivisceral cavity, the Trematoda 

 were placed by Cuvier into a separate division of Entozoa, un- 

 der the name of Vers Intestinaux parencJn/mateux, along with 

 the Tceniada and Acanthocephala, in which no alimentary 

 canal is present. By Owen, for the same reason, they are in- 

 eluded in a distinct class, under the name of Sterelmintha. 



The Distoma kepaticum (fig. 40) may be taken as the type 

 of the Trematoda. It is the common ' Liver-fluke ' of the 

 sheep, and inhabits the gall-bladder or biliary ducts, giving 

 rise to the disease known as the ' rot.' In form it is ovate, and 

 flattened on its two sides, and it presents two suctorial discs, the 

 anterior of which is perforated by the aperture of the mouth, 

 whilst the posterior is impervious. Between the suckers is 

 the ' genital pore,' at which the efferent ducts of the repro- 

 ductive organs open on the exterior. A branched water- 

 vascular system is present, and opens posteriorly by a small 

 aperture. The alimentary canal bifurcates shortly behind the 



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