504 DEVELOPMENT AND GEOWTH OF T^NIA SOLIUM. 



usually measures about 1 mm. But this length is almost equally 

 divided between the two joints, one of which effects the connection 

 with the wall of the bladder to which it is attached perpendicularly, 

 while the other enclosing the enlarged blind end of the head-cavity 

 is bent to the side, almost at a right angle. The receptacle sur- 

 rounding the head has departed from its previous form in being 

 laterally protruded before the bend of the head. Breadth and height 

 measure somewhat uniformly 0'5 mm. The space running up the 

 axis of the papilla is but slightly enlarged, except at its blind end. It 

 has for the most part the appearance of a narrow canal, and in the 

 basal portion is not straight but of zigzag form, and therefore longer 

 than the linear distance between its ends. This form is obviously the 

 optical expression of a folding of the wall of the papUla, and is some- 

 times so inconstant that the application of pressure and simultaneous 

 elongation and broadening out of the canal cause it to disappear. 

 The opening of the canal on the bladder appears usually as a transverse 

 slit, whose borders protrude in the form of two narrow lips. The 

 calcareous bodies which surround this slit in a circle have increased 

 in number, and have also spread themselves over the root of the head- 

 papilla. Otherwise, the structure and histology are much as before. 

 Internally, one recognises here and there the subsequent longitudinal 

 vessels, but always only in fragmentary fashion, since they are easily 

 destroyed by*pressure. 



The early bending of the head-papilla here described is, as far as I 

 know, an exclusive characteristic of Cysticercus cellulosce. In the other 

 bladder-worms known to me the head retains its originally straight 

 position, almost unchanged, until the formation of the suckers and 

 circle of hooks. The flexure, if it occur at aU, does not begin till 

 later, when the neck increases greatly in length after the completion 

 of the head. In the bladder-worm of the pig this elongation occurs at 

 an earlier period than usual. ^ 



In spite of the marked curvature of the head-papilla, no proper 

 head is yet to be found. The process is indeed enlarged terminally 

 like a club, and the internal canal is markedly enlarged, but there 

 are as yet no traces of suckers or hooks, which are the most important 

 characteristics of the tape-worm head. The next stages, however, 

 result in the formation and development of these organs. 



We may pass over the details of the process with a simple refer- 

 ence to our former discussion (p. 351). There we saw how the suckers 

 originated round the enlarged cavity, but the hooks and rostellum 



* Perhaps the Cysticercus longicoUis also resembles the bladder-worm of the pig. At 

 least one finds specimens of this worm in which the head-papilla is wound into a spiral, 

 which bears suckers even before the hooks are perfectly developed. 



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