CESTODA 



109 



repeated so that a single parasite keeps on casting off portion after 

 portion, each charged with ova capable of developing. In this way 

 a Cestode firmly lodged in the alimentary canal of its host can 

 produce an almost indefinite number of eggs. 



In the more primitive Cestoda the egg-cases, which as in Trema- 

 toda contain an ovum and a multitude of yolk-cells, escape from 

 the detached progiottis through the opening of the uterus into 

 water. In a short time a larva hatches out, consisting of an outer 

 layer of ciliated cells surrounding a solid internal mass which deve- 

 lops six chitinous hooks. After swimming in the water for a short 

 time the larva is swallowed by an aquatic animal, loses its outer 



FIG. 47. 



Diagram of a ripe progiottis of Taenia solium. 

 x about 10. 



From Cholodkowsky 



1. Longitudinal water-vascular canal. 2. Transverse water-vascular canal. 

 3. Vasdeferens. 4. Vagina. 5. Ovary. 6. Yolk-gland. 7. Shell- 

 gland. 8. Uterus. 9. Testes. 10. Longitudinal nerve. 



layer of cells, thus exposing the hooks and becoming what is now 

 termed an onchosphere. In the more modified Cestoda the uterus 

 has no external opening, and the eggs escape from the progiottis only 

 by decay. The remains of the progiottis, including the eggs, are 

 swallowed by some animal, and soon after an onchosphere hatches 

 out, no cilia being formed. In every case the onchosphere bores 

 through the alimentary canal of its host, and is carried by the 

 blood-stream to a suitable spot in the tissues where it fixes itself. 

 Once fixed, the larva increases very much in size and generally 

 becomes hollow so as to resemble a bladder. An infolding or 



