VARIATIONS IN THE FORM OF THE HOOKS. 



581 



hominis into E. altricipariens, and E. veterinorum into E. scolecipariens. 

 He supports his opinion not only by a reference to differences in the 

 process of proliferation, but also by the statement that both species 

 present decided differences in number, size, and in the form of the 

 hooks. Echinococcus scolecipariens has, according to Kiichenmeister, 

 twenty-eight to thirty-six hooks of a stouter form, while E. altrici- 

 pariens has between forty-six and fifty-six, which differ markedly 

 from those of the latter in their slender and elegant structure. 



If Kiichenmeister were correct in these observations, hardly any 

 objection could be raised against the specific nature of these two forms. 

 But such is not the case : not only does the number of hooks vary in 

 both of them between the two extremes (especially those which lie 

 further from each other, as in most of the other bladder- worms), but 

 the form of the hooks by no means exhibits any decided differences. 

 In comparing the two species, we ought always to go back to 

 equivalent stages of development. But if, like Kiichenmeister, we 

 neglect to do so, and compare the tape-worm taken from E. veterinorum 



FIG. 315. Echinococcus hooks ( x 600). (A.) Of Echinococcus 

 veterinorum; (.) Of Tcenia echinococcus in the third week; (C.) Of 

 adult Tcenia echinococcus ; (D.) The three different hook-forms drawn 

 one within the other to show their gradual changes. 



with the rudimentary head of E. hominis, we shall indeed see very 

 striking differences of form ; but, according to my observations, these 

 differences are to be explained by the fact that the Echinococcus-hooks 

 only attain their full development during the transformation of the 



