36 THE TAPE-WOEM. 



and the two outermost hooks are alike. 1 If one of these embryos 



is set free (which can be effected by care- 

 fully crushing the eggshell between two 

 plates of glass), without destroying the 

 living tape-worm embryo, its various 

 movements may be examined under the 

 microscope. It draws its round body 

 together, and enlarges and contracts its 

 transverse diameter, and by this operation protrudes, first in 

 front and then at the sides, the six little hooks from that end 

 of its body which, from these hooks being situated there, I shall 

 call the fore part. The observer can readily understand how, by 

 such movements, the excessively minute cestoid embryo succeeds 

 in boring its way into the moist and tender soft parts of other 

 animals and in traversing their interior in all directions. 



When the cestoid embryos have, by immigration and subse- 

 quent encysting, lodged themselves in an animal by whose 

 means they will eventually become introduced into the alimentary 

 canal of one of the Vertebrata, and so reach the last stage 

 of their development, a remarkable metamorphosis takes place 

 by which they pass from the condition of embryos into that of 

 scolices. In the interior of the embryo an organ is developed 

 which gradually assumes the characters of the head of a cestoid 

 worm, and always resembles that of the particular species from 

 which the embryo has been produced. When once the head of 

 the cestoid is fully formed it may become extruded from the 

 interior of the body, and the entire worm then constitutes a scolex. 

 The whole of this process of scolex-development may be justly 

 compared to an internal budding. 



According to the view of the earlier helrninthologists, the 

 scolices consist of the head end of a cestoid worm, out of whose 

 posterior extremity the proper body is subsequently developed. 

 In regard to the organization of the scolices, it must be par- 

 ticularly noted that they possess no oral aperture, and are only 

 nourished by the absorption of fluids through the surface of their 



Fig. 18. The embryo of Tcenia crater if or mis. The six hooklets are formed upon 

 three different types ; b, c, d, represent the three kinds more highly magnified, b. One 

 of the two uppermost, e, one of the two median, and d, one of the two outermost 

 hooklets. 



1 See my description of these hooks in Burdach's ' Physiologic,' Bd. ii, 1837, p. 204. 



