52 PLATYHELMINTHES. 



TREMATODA. 



Trematodes are flat worms which lead a wholly parasitic 

 life, but which have retained, to a greater or less degree, those 

 organs that characterize free-living animals. Some Trematodes 

 are parasitic upon the outside (or ectoderm) of other animals, 

 and are hence called ectoparasites. 



HAEMATOLOECHUS (DISTOMUM). 



This form is found as a parasite in the lungs of frogs. In 

 some localities a large proportion of the frogs are infested and 

 several specimens are frequently found in one frog. The host 

 of the asexual generation of this species is not known, but in a 

 closely allied species the asexual generation lives in the pond- 

 snail. The living worm is cylindrical and pointed at both an- 

 terior and posterior ends. With a low-power objective note: 



1. The anterior sucker, surrounding the mouth. 



2. The ventral sucker, near the middle. 



3. Do you find eyes f 



4. The alimentary canal. 



(a) Mouth. 



(b) The muscular pharynx. 



(c) Soon after leaving the pharynx the intestine divides 

 into two equal branches, which pass, one on the left and one on 

 the right side, to near the end of the body. These intestinal 

 branches do not send out lateral branches as they do in Bdel- 

 loura. 



The Water-vascular System. — A small opening will be found 

 at the posterior end of the body from which a duct passes forward 

 in a median position to a point a little posterior to the median 

 sucker. Here it divides and sends a branch on either side of 

 the worm to near the anterior end. 



The Nervous System. — This is difficult to see, but in a 

 mounted specimen a small, deeply stained mass, the cerebral 

 ganglia, may be visible on either side of the pharynx. Three 

 pairs of longitudinal nerves pass back to near the posterior end 

 of the body. 



