Variation in Hooks of Dog-Tapeworms 9 



thus account for a very slight amount of the variation in the 

 magnitudes of the chord lengths as measured in the drawing. 

 The branches of the ventral root will be either exactly opposed 

 to each other in position, or one branch will slant forward and 

 the other backward. 



This is partly due to the tilting of the hook, though not en- 

 tirely. In the extreme cases it is merely a matter of judgment 

 as to what is the proper point from which to measure, but usu- 

 ally the anterior branch was used. These two branches are often 

 of unequal length, and in such cases the longer branch was used 

 in measurement. The cleft between the two branches is exceed- 

 ingly variable in its depth, and may be, in fact usually is, re- 

 duced to a mere furrow or shallow depression extending from the 

 base of the root on either side over the end (pi. I, figs. 2, 3, 4). 

 In other hooks the cleft extends into the root fully two-thirds of 

 its length (pi. I, fig. 5). 



The general outline of the small hooks also varies from an 

 even and continuous to a rough and broken margin. A deep 

 notch occurs immediately behind the ventral root in some cases 

 and is followed by an abrupt elevation that gradually inclines 

 backwards for a short distance to a level with the general outline 

 of the dorsal root. Usually this notch is absent, though there is 

 always the elevation noted. The end of the dorsal root is usu- 

 ally club-shaped, but is occasionally drawn out to a point (pi. 

 Ill, figs. 14, 15). The dorsal line of the hook may or may not 

 have an angle at the origin of the blade or prong. The majority 

 of the small hooks present a very even outline for the dorsal 

 root. 



large hooks of Taenia serrata 



The large hooks of Taenia serrata present as irregular an out- 

 line as the small ones. The ventral root varies in shape, size, 

 and direction of its slant, but is not bifid. The anterior margin 

 may pass evenly into the lower margin of the blade, but quite 

 frequently there occurs a notch at the base of the root. The 

 posterior margin may also present the same differences as the 

 anterior. Different authors describe the ventral root as heart- 



199 



