172 



MANUAL OF ZOOLOGY. 



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 larva are 

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

 " cystic." 



From the absence of a perivisceral cavity, the Trematoda 

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

 der the name of Vers Intestinaux farmchymateux, along with 

 the Tceniada and AcantMcephala, 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. 49) may be taken as the type of 

 the Trematoda. It is the common " Liver-fluke " of the sheep. 



Fig. 49. — Trematoda. i. Distoma he^aticuntt the " Liver-fluke," showing the branched 

 aKmentary canal. 2. Anterior extremity of Distoma lanceolaiuiti. a Anterior 

 sucker; ^Posterior sucker; c Generative pore; <^ CEsophagus ; ^ Alimentary canal. 

 (After Owen.) 



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. The larvae <^ Distoma 

 are tailed or " cercariform," and are found in the interior of 

 fresh-water snails. 



