of an alternation of generations as seen in the jelly fishes. 



1. Egg. 



2. Morula. 



3. Ciliated larva. 



5. Cercaria (nurse, scolex). 



6. Encysted Cercaria (Proglottis). 



7. Distoma (Proglottis). 



The Cercaria echinata, living in snails which are oaten by duck-, 

 have been shown by St. George to develop into the adult Distoma 

 in the body of that bird. It is generally the case that those Dis- 



the larval state in animals which serve as food for higher orders. 

 Thus the Bucephalus of the European oyster passes in the en- 

 cysted state into a fish (Gasterostomum), which serves as food for 

 a larger fish, Belone vulgaris, where the cysts of the same worm 



Distoma hepaticum, the liver fluke, sometimes occurring in man, 

 is thought by Dr. Willemoes-Suhm to begin its existence as Cer- 

 caria cystophora, parasitic on a species of Planorbis. 



Development of the Cestodes. In the tape worm there is no ali- 

 mentary canal, the liquid food being absorbed from the juices of 

 its host through the walls of the body. The head is armed with 

 suckers, hooks or leaf-like soft appendages, while the body is sub- 

 dh ided usually into a great number of segments, each containing 

 an ovary and male gland. While the Turbellaria possess a pair 

 of nerve-ganglia, the Cestodes are not known positively to possess 

 any trace of a nervous system. 



E. Van Beneden shows that the egg is formed by two glands, 

 one of which (the germogene) forms the nucleus and nucleolus, 

 while the other (vitellogene) forms the yolk. Development begins 

 very probably as in the Trematodes, by multiplication by <1iwsh>i> 

 of the nucleus (germinative cell). In the eggs of Tania bacil- 



