506 



DEVELOPMENT AND GROWTH OF T^NIA SOLIUM. 



in breadth and in length, and that so quickly that the enveloping 

 receptacle rapidly acquires double or triple its former diameter. 



In spite of this increase in size, the head-process still retains its 

 early position inside the receptacle. The flexure, however, gradually 

 passes into a spiral curvature, and the head is more or less raised by 

 the growth of the succeeding portion. With increased age and length, 

 the spiral becomes more perfect, so that one finds specimens where the 

 head-process exhibits one and a half turns and more. In its unrolled 

 state the body of the worm measures about 10 mm. long by almost 

 2 mm. broad. In order to understand how so large a body can find room 

 inside the receptacle, we must remember that the body of the worm is 

 not only spirally coiled, but is also abundantly wrinkled and folded 

 transversely. There are but few bladder-worms where the head- 

 process is equally folded. In longitudinal sections one sees (Fig 286) 

 the boundary on either side raised in long, thin, comb-like processes, 

 interlocking with each other like fingers, and almost filling up the 

 internal cavity. Here and there the folds are also beset with small 

 elevations. Still more striking is the fact that the head-process of 

 the adult bladder- worm seems to stand in no direct connection with 

 the bladder. 



Fig. 284.— The 

 common bladder- 

 worm of the pig, 

 with invaginated 

 head. ( x J.) 



Fig. 285.— The 

 same, with evagi- 

 nated head. ( x 2. ) 



Fig. 286. — Longitudinal sec- 

 tion through the head-process 

 of the same. ( x 40. ) 



Instead of being attached to it, as before, at the margin of the invagi- 

 nation, the latter is now separated off, so that its basal end protrudes 

 freely into the head-cavity, like the proboscis of a gasteropod. The 

 connection between the head-cavity and the invagination opening is 

 thus effected by a sort of outer cavity, including mainly the basal 



portion of the head-process. It does not remain exclusively confined 

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