UNSEGMENTED WORMS. 189 



in the water. In order to develop it must find its way into the tis- 

 sues of a particular species of pond-snail. Here it- undergoes a 

 series of rapid non-sexual reproductions, each time infesting the 

 tissues of the snail more completely (Fig. 54, D-F), and ultimately 

 bringing about its death. After reaching the cercaria stage (Fig. 

 54, G), it is free-swimming for a short while and then becomes 

 encysted so as to enable it to await the time when it will be taken 

 into the body of a sheep again, when it drinks the water. The 

 cyst is then dissolved, the animals make their way to the liver and 

 develop into the adult fluke (Fig. 54, I). It will be seen that this 

 cycle illustrates both a metamorphosis and an alternation of genera- 

 tion. The student will notice how closely the organism is adapted 

 to the life both of the sheep and the snail. Any failure to meet this 

 special adaptation means death, and millions do fail to make connec- 

 tion. The tremendous power of reproduction is all that saves the 

 species. 



207. The third class (the Cestodes or tape-worms) are also para- 

 sitic and in a general way resemble the trematodes in the complexity 

 of their life. In the adult they are found attached to the wall of the 

 stomach or intestine of the host and consist of a long (often many 

 feet in length) chain of bodies (segments or individuals) . The oldest 

 segments are furthest from the "head," or attached end (Fig. 55). 

 Each of these units is a perfect sexual individual, having organs 

 producing both eggs and sperm. The eggs are fertilized in the 

 body and remain there during early development. They grow to 

 such an extent that they gradually destroy the other organs and 

 the oldest of the segments are merely cases, full of young embryos. 

 These break off and escape at this stage. As this happens new 

 segments are being formed just behind the "head," and thus the 

 colony keeps its length. This will continue indefinitely as long 

 as the head holds to the wall of the intestine and proper nourishment 

 is supplied. It is provided with suckers or hooks for this purpose. 

 There is no mouth, no digestive tract, no circulation. There is a 

 nervous centre in the head, excretory tubes running the length of 

 the colony, and very complex reproductive organs in each segment. 

 There are numerous muscles, but there is no real locomotion. 



In the life history the mature cases break away from the chain 

 and escape to the outer world from the digestive tract of the host. 

 If they are not taken up by some other animal this is the end of them. 

 If swallowed by the proper host the embyro case is dissolved and 

 the embryos escape and soon bore through the wall of the digestive 

 tract into the circulation or directly into the tissues. Here they 



