ON PARASITES. 35 



proglottides fall into the water they swell up and rupture, strewing the 

 eggs about. Thus in various ways the eggs are distributed in the 

 outer world, some exposed, others still in the proglottides. If a pro- 

 glottis chance to rest upon a morsel of food of any animal, it is swal- 

 lowed, and passing into the stomach the outer envelope of the eggs is 

 digested off. In this case numerous cystic worms will be generated in 

 the same animal. If single eggs are swallowed there will only be one 

 cystic worm. 



Having discharged its contents the function of the proglottis ceases. 



The Six-Hooked Embryo. 



The egg having arrived in the intestine of its destined host, either 

 singly or in company with others, its shell ruptures and the six-hooked 

 embryo issues forth. 



ITS ANATOMY. 



A globular naked vesicle, varying in size from 0, 022 to 0, 05 mim. 

 without internal organs, and provided on its anterior extremity with 

 six, (occasionally four,) microscopic booklets, whose points are direct- 

 ed downwards. The rupture of the egg shell takes place chiefly in 

 the stomach. 



PHYSIOLOGY. 



It is nourished by imbibition, — is not acted upon by the intestinal 

 juices of its host, — is capable of voluntary motion. 



ITS DESTINY. 



Whether in the stomach or intestine, after the escape from the 

 shell, the embryo fastens by means of its booklets upon the structure 

 of its host, and according to Van Beneden, it brings together the cen- 

 tral pair of booklets like a wedge and by thrusting and twisting 

 begins to force them forward. Having thus made some progress, it 

 assists itself by the use of the two lateral pairs of booklets. By 

 this boring movement it penetrates into the tissues and into the por- 

 tal vessels. Having thus entered the circulation, it is carried to 

 distant parts of the body. After a passive migration in this manner, 

 it sticks fast in a capillary somewhere, and again commences an active 

 migration or encysts itself there. Having migrated to the locality 

 which is favourable for its future development it fixes itself.* Having 



• The embryos of some species migrate actively only. 



