18 ENTOZOA. 



Gyrodadylus, etc.), many of whicli are ectozoa rather than entozoa 

 in the ordinary sense of the terms. No doubt, on the other hand, 

 many immature forms were described (from the older authors) as 

 distinct species ; yet, notwithstanding, when we take into considera- 

 tion the additions which have been recently made — especially by 

 Molin of Padua — and also the probably much larger number of 

 forms which remain undiscovered, it becomes evident that at the 

 very lowest possible estimate we may assume the order to comprise 

 400 species. These animals may be classed as follows : — 



/ Mbnostomidce (Family I.) 

 I Distomidcs (Family II.) 

 Tkematoda } Tristomidoe (Family III.) 

 I Polystomidce (FamUy IV.) 

 I CtyrodacU/Udm (Family V.) 



The arrangement here adopted is very similar to that offered 

 in my " Synopsis," but I have since thought that the Monostomes, 

 though essentially flukes, cannot consistently be placed under the 

 Distomes — ^unless, indeed, we are prepared to abandon altogether 

 this very simple and convenient mode of classification. The 

 number of suckers {o-TcafiaTa) forms a good, sahent, family character, 

 but in the case of the Polystomes a particular number need not be 

 insisted on. Provisionally, at least, I prefer to let the group stand 

 as above, where it will be seen that I have also elevated the Gyro- 

 dactyles into a distinct family. 



Monostomidce. — The members of this family are distinguished 

 by the presence of a single pore or oral sucker, which is situated 

 at the anterior extremity of the body. Usually the digestive 

 system consists of two rather capacious ccecal tubes given off by 

 the oesophagus and terminating on either side of the middle line, at 

 the lower part of the body. In some species, however {Monostoma 

 mutabile and M. flavum), the two alimentary tubes are blended 

 together below in an arcuate manner. The sexes are always com- 

 bined, the disposition of the reproductive organs closely resembling 

 that found to obtain in the hermaphroditic Distomes. A well- 



