EVAGINATION OF THE HEAD. 353 



remains inverted in the maternal bladder (Fig. 191). But the develop- 

 ment of the bladder-worm is not yet quite complete. The head, 

 which was at iirst only a little distant from the bladder-wall, becomes 

 ever further removed by the elongation of its basal portion ; thus there 

 arises in course of time a distinct vermiform body, extending in 

 cylindrical form between the upper narrow neck-like portion of the 

 head and the bladder. Both head and body lie still within the 

 receptacle (Fig. 197). The succession of layers corresponds with that 

 of the head ; the surface covered with cuticle is inwards, next the 

 cavity. The musculature is more strongly developed than in the 

 head, and the number of calcareous bodies is usually very con- 

 siderable. 



The older the bladder-worm, the more this body grows ; it acquires 

 numerous cross wrinkles and folds, which usually extend far into the 

 canal-like space, and it squeezes itself together in the extended recep- 

 tacle as far as the space will permit. As a rule, the head takes up a 

 lateral position — which we have already noted in the bladder- worm 

 of the pig — as a natural consequence of the growth in length and 

 early bending of the head (p. 346). The connection with the receptacle 

 presents no special difficulty in the way of this change of position, 

 for the connective substance which has been produced is soft and 

 extensible, and permits many alterations (Fig. 197). If the receptacle 

 be unable completely to contain the contents, then the end of the body 

 which is next the bladder is extruded (Fig. 198) from the opening 

 of the head-cavity, so that the former inner surface comes to be 

 external. Sometimes the whole of the body and head are thus evagi- 

 nated, so that the bladder-worm becomes (Fig. 199), indeed, like a 

 tape-worm with an incompletely jointed body and an attached 

 " caudal bladder" {Tcenia visceralis). 



In this evagination the musculature of the bladder has probably a 

 share as well as that of the receptacle. The pressure which the former 

 can exercise affects primarily only the enclosed fluid, but this is trans- 

 mitted in all directions, and is finally most influential where the 

 resistance is least — namely, at the position of the evaginating head. 

 The process is essentially the same as we formerly (p. 311) described 

 in the protrusion of the penis from the so-caUed " cirrhus-pouch," 

 which stands to the cirrhus in much the same relation, even anatomi- 

 cally, as the receptacle to its contents. The retraction of the protruded 

 structure can of course be effected only by special muscles. 



Though the whole head-mass of the cystic worm has been evaginated, 

 this does not prevent some of the individual parts remaining intro- 

 verted. This is particularly true of the head proper, which protrudes 

 (Fig. 193) from the bottom of the appendage, and not unfrequently 

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