PLATYHELMINTHES 223 



water, and in districts in China and Egypt where the disease is com- 

 mon they swarm. Bathing, washing or drinking the infected water 

 allows the cercaria to enter the final host. The cercariae penetrate the 

 skin with great rapidity and, entering the blood system, make their 

 way to the abdominal veins where they become mature. The disease 

 can be prevented by strict sanitary measures in regard to water, and 

 it can be cured by the administration of compounds of antimony to 

 infected patients. That the disease is a very old one in Egypt is shown 

 by the discovery of Schistosoma eggs in the kidneys of mummies of 

 the twentieth dynasty (i 250-1000 B.C.). 



The hatching of miracidia from the egg of Schistosoma is dependent 

 on the dilution of the urine by fresh water and this serves to emphasize 

 the fact that the stages in the life history of all parasites are ultimately 

 connected with environmental conditions. The egg ofFasciola hepatica 

 does not hatch unless the pYi of the water in which it is deposited is 

 below 7-5, the optimum point apparently being about pYL 6-5. If 

 the eggs are kept in water more alkaline than pYi 7-5 the embryo 

 remains within the shell and eventually dies. 



The identification of a cercaria with an adult is a task which requires 

 great patience, and many cercaria are known which have not been as 

 yet connected with an adult. Almost any mollusc, if dissected care- 

 fully under a hand lens, will provide specimens of rediae and cercariae, 

 although infected specimens may be more common in some localities 

 than in others. The tail of a cercaria is often an elaborate structure. 

 Some have rings and chitinous stiffenings, while the well-known 

 Bucephalus larva of Gasterostomum is a cercaria with a forked tail 

 (Fig. 161). 



Class CESTODA 



The Cestoda may be defined as endoparasitic Platyhelminthes in 

 which the enteron is absent and the ciliated ectoderm has, in the adult, 

 been replaced by a thick cuticle. In the parenchyma lime cells occur 

 (see Fig. 162). Proglottides are usually formed. 



The Cestoda as a group have felt the influence of the parasitic habit 

 more than the Trematoda. They have dispensed altogether with a gut, 

 there is no mouth, and they absorb their food through the skin. As 

 they live always in the alimentary canal of vertebrates they are con- 

 veniently situated for this purpose and the amount of food available 

 to them probably counterbalances the difficulties attendant on dis- 

 pensing with the usual method of digesting and assimilatiiig food. 

 The ectoderm cells have sunk into the parenchyma after secreting a 

 cuticle as in the Trematoda, but this cuticle is thicker and divided into 

 layers. Immediately beneath the cuticle are the longitudinal muscles. 

 The circular muscles are incomplete at the edges. In transverse 



