TREMATODA. 31 



The second type is that of the Distoma (the 

 Trematoda digenesia of Van Ben den), and comprises 

 all the trematode worms which have not more than 

 two suckers. Their development is indirect, the 

 embryo bearing no resemblance to the parasite which 

 has produced it. All of them, at the adult period, 

 live in the interior of vertebrated animals. 



The distoma never attain to a very large size ; 

 they are usually flat, and present a fohaceous 

 appearance. 



Their movements are very limited, and are prin- 

 cipally accomphshed by means of their suckers, 

 similarly to the movements of leeches. Most ire- 

 quently one of the suckers is situated at the anterior 

 extremity, and constitutes at the same time the 

 mouth ; the other which is imperforate, and simply 

 serves to fix the animal in position, is situated on the 

 ventral surface, at some distance from the anterior 

 sucker, or even at the caudal extremity ; it is not 

 found m every species. 



With the exception of two species, all the dis- 

 toma are hermaplirodite. 



The ovum is sometimes completely developed 

 within the oviduct. The embryo which it encloses is 

 commonly furnished with vibratile cilia, and is occa- 

 sionally armed with hooks ; as has been already 

 stated, it always differs greatly in its form and 

 organisation from the individual which has given 

 birth to it. 



It is by altermate generation that the type of the 

 individual which has produced the embryo re-appears ; 

 this latter, similar to an infusorial worm, has no dis- 

 tinct mternal organs ; it is generally covered with 



