14 ENTOZOA. 



CHAPTEE II. 



TRBMATODA. 



The Trematoda, or flukes — Aspect and habits — Distribution in mammals, birds, reptiles, 

 and fishes — Estimated number of species — Classification of the order — The family 

 of Monostomes — Their development and genera — The Distomes — Then- anatomical 

 structure and development — Yan Beneden's investigations respecting Distoma 

 iniUtare — Pagenstecher's researches — Genera. 



In accordance witli the plan proposed, I come now to speak of tlie 

 second order of Helminths, which, under the old arrangement, 

 would be the first order of the Entozoa proper. This order, the 

 Trematoda, as the G-reek word [TprjfA.aTcoSij'i) from which it is 

 derived indicates, is characterized by the possession of certain 

 pores or openings ; not that the presence of these pores, however, 

 is altogether distinctive, but, when these characters are associated 

 with other structural peculiarities, then their ordinal value becomes 

 sufficient and, indeed, very strongly pronounced. 



All the animals included in the order have soft, roundish, or flat 

 bodies, their contained visceral organs being lodged in the general 

 parenchyma of the body. This latter character, it will be remem- 

 bered, we also found uniformly present in the other sterelminthic 

 order, namely, the Turbellaria. 



Aspect. — The ordinary aspect of these creatures is not such as 

 would, at first sight, recommend them to the attention of the 

 general observer ; yet those who study them patiently will not fail 

 to be struck by their beauty. They are small animals, usually 

 visible to the naked eye, and seldom attaining any very significant 

 bulk; a few of the minuter forms scarcely exceeding the 1-1 00th 



