THE CCELOMATA INVERTEBRATA 



199 



hence known as the onchosphere or hexacanth embryo. No further 

 development can be undergone until it is swallowed by the pig, whose 

 digestive fluids dissolve off both the egg shell and the cuticle, releasing 

 the tiny six-hooked embryo. This then bores its way through the 

 wall of the gut largely by the aid of its hooks and penetrates a small 

 blood vessel. In the blood stream it is carried about until it reaches 

 its destination, which in T. solium is usually the voluntary muscles. 

 It increases in size fairly rapidly, losing its hooks and becoming 

 inflated with a fluid substance until it forms a thin-walled hollow 

 vesicle about 5 mm. in diameter, known as the proscolex. At first 

 this is uniform all round its periphery, but soon it thickens at one 



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FIG. 64. Diagram of the development of Tcenia, adapted from Leuckart. 



I., hexacanth embryo ; II., early proscolex ; III., IV., and V., anterior end of proscolex in different 

 stages of development, more highly magnified ; V., beginning of evagination ; VI., complete 

 evaginated scolex with caudal vesicle. 



point. The thickening invaginates and is the primordium of the- 

 head of the future worm. At its inner end the invagination dilates 

 to form a hollow vesicle, within which are produced the suckers, 

 rostellum and spines, in fact a miniature head only inside out. By 

 this time it has increased considerably in size, being oval and about 

 12 by 8 mm. in diameter, and it is known as the bladder worm or 

 Cysticercus. It consists of a large vesicle, the proscolex or caudal 

 vesicle, as it may now be termed, filled with fluid, and projecting into 

 this is the introverted scolex. This marks the limit of its develop- 

 ment in the pig, and it now remains in a passive condition embedded 

 in the muscular tissue which secretes around it a secondary cyst of 



