Variation in Hooks of Dog-Tapeworms 3 



In the study of the hooks a Leitz compound microscope was 

 used with a No. 2 ocular and No. 7 and No. 8 objectives, magni- 

 fying the projected drawing 416.5 and 568 times respectively. 

 By means of the camera lucida, each hook was projected and 

 drawn, care being taken to have the same relative position and 

 adjustment at all times of the tube length, camera lucida, and 

 drawing board. The No. 7 objective was used in examining the 

 hooks of T. serrata, and the No. 8 objective for the hooks of T. 

 serialis. All measurements were made on the projected draw- 

 ings, with a scale prepared by projecting a stage micrometer, 

 making the magnification correspond to that of the projected 

 hooks. This method of measurement is more accurate than 

 measuring with an eye-piece scale, as with the latter it is prac- 

 tically impossible to locate definitely constant points on the hooks, 

 since it is necessary to manipulate both the eye-piece scale and 

 the mount in making the different measurements. 



The distance between two parallel lines placed one at each ex- 

 tremity of the hook, and perpendicular to a line connecting the 

 two extremities, is taken as the total length of the hook and des- 

 ignated as character a (see fig. 9, pi. III). Character c, the an- 

 terior chord length, is the distance from the point of the prong 

 to the point of tangency on the extremity of the ventral root, of 

 a line drawn from the point of the prong. Character b, the 

 posterior chord length, is the distance from the point of tangency 

 on the ventral root to the extremity of the dorsal root. Charac- 

 ter d is the length of the perpendicular from the anterior chord 

 to the highest point of the curve of the prong. 



It is readily apparent from the figure that the anterior and 

 posterior extremities of the hook and .the tangency point on the 

 ventral root are the constant points for measurement in all the 

 hooks. It is necessary to select constancy points of this general 

 character because of the variation in the shape and relative posi- 

 tion of the different parts of the hook. 



The four measurements of the hook are taken for the purpose 

 of correlation or comparison of these characters with regard to 

 their relative deviation from a mean magnitude to be determined. 

 For instance, character a is correlated with character b and a 



193 



