ANNULOIDA: SCOLECIDA. 



I6 5 



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

 der the name of Vers Intestinaux parenchymateux, along with 

 the Tceniada and Acanthocephala, in which no alimentary canal 

 is present. By Owen, for the same reason, they are included 

 in a distinct class, under the name of Sterelmintha: 



The Distoma hepaticum (fig. 47) maybe 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 ante- 

 rior 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 reproductive 

 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 mouth, the 

 two divisions thus produced giving off numerous lateral diver- 

 ticula, and terminating posteriorly in blind extremities. The 

 nervous system consists of a ring round the gullet, giving off 

 filaments both forwards and backwards. 



Fig. 47. Trematoda. i. Distoma hepaticum, the " Liver-fluke," showing the branched 

 alimentary canal. 2. Anterior extremity of Distoma lanceolatum. a Anterior 

 sucker ; b Posterior sucker ; c Generative pore ; d (Esophagus ; e Alimentary canal. 

 (After Owen.) 



In Distoma lanceolatum (fig. 47, 2) the intestine has not the 

 ramose, complex character of that of D. hepaticum. On the 

 other hand, the alimentary canal, after its bifurcation, is con- 

 tinued on each side of the body to the posterior extremity 

 without giving off any branches on the way, and it terminates 

 simply in blind extremities. 



Diplostomum, in its essential characters, does not differ 

 much from Distoma; but it is found living gregariously in the 

 vitreous humour and lens of the eyes of certain fresh-water 

 fishes, such as the common Perch. 



