METAZOA VERMES. 323 



CESTODES. 



All Cestodes are internal parasites and illustrate ex- 

 treme specialization by a profound modification of some 

 organs and by the total suppression of others. * 



The important fact that there is no mouth and no 

 digestive system in either the young or the adult Cestode 

 proves how far removed these parasites are from the 

 primitive ancestral forms. 



The tape-worm, Taenia solium, found in the intestine of 

 man, is an instructive example. 



Its eggs do not develop in water nor in earth, like those 

 of the tape-worm's remote ancestors, but within the body 

 of another animal, the hog. Thus the environment has 

 changed throughout the whole life of the worm, and with 

 this complete change in physical surroundings there has 

 followed a complete change in structure. The eggs, 

 which may be seen in the little bottle at the bottom of 

 the jar (No. 800), leave the intestine in faecal matter 

 and are swallowed by swine which are often kept in un- 

 clean places. The young of the tape-worm originally 

 described as Cysticerciis cellulosae Auct. are bladder-like 

 and for this reason are often called bladder-worms. The 

 figure (PL 798, fig. i) shows the bladder with just the 

 beginning of the head. In fig. 2. the head is turned in- 

 ward, and in fig. 3 outward ; between the head and blad- 

 der the neck has developed. In time the head or scolex 

 of this larva is provided with hooks and suckers. (No. 

 799 is the young of another species of Taenia which 

 is found in the liver of the rat. These are swallowed 

 by the cat in whose intestine they develop.) 



The larvae of Taenia solium bury themselves in the 

 flesh of the hog, which is often eaten by man in a raw or 

 half-cooked condition. In this new situation the worm 

 develops rapidly. The bladder is cast off. The head 

 (PI. 798, fig. 4) has a double circle of hooks and four 



