8 Earle C. Stevenson 



ing could be ascertained, the removal of the parasites would be 

 much easier, as their attachment to the intestinal wall would be at 

 that time much less firm. 



Whatever be the cause or causes of the difference in the num- 

 ber and size of the hooks in different individuals from the same 

 and different dogs, the fact that there is a difference in the varia- 

 tion in the size of the hooks on some individual heads amounting 

 to over one-half of the total difference between hooks of the dif- 

 ferent heads, is proof that a factor other than supposed processes 

 of moulting, or difference in age, must be considered in studying 

 the growth of the hooks. 



It is evident that Leuckart regarded the hook as fully developed 

 in the Cysticercus stage. Whether or not the hooks grow after 

 entrance of the Cysticercus into the alimentary canal of the dog is 

 a question directly influencing the data used in this paper. If a 

 constant factor, such as age, affects the magnitudes of the hooks, 

 and is not considered in measurements taken, the results in aver- 

 ages are unreliable. It is therefore assumed that practically con- 

 stant normal characters are under consideration. 



DESCRIPTION OF THE HOOKS 



Taenia serrata. — O. Deffke (1891) gives a fairly typical draw- 

 ing of the hooks of Taenia serrata, but observation will show 

 wide deviations from this figure. This is also true of all the 

 figures I have found in works of different authors. In the small 

 hooks of Taenia serrata the angle of the arc and position of the 

 dorsal and ventral roots affects all the magnitudes ; some of the 

 small hooks have the point of the prong bent sharply down- 

 ward, while others have a variation in the angle of the prongs 

 of hooks of the same a magnitude. 



These peculiarities make a difference in the chord magnitudes 

 b and c. The ventral root, which is bifid, will have the prongs 

 slanting more forward in some cases than in others, but in all 

 books these branches extend laterally, as is seen in Plate II, fig. 

 Sa. This lateral projection of parts of the ventral root will cause 

 the hook to tilt either forward or backward when on its side, and 



198 



