110 ANIMAL PARASITES. 



of all the stems lie in two circles at a considerable distance from 

 each other. The form will be best seen from the figures. (PI. IV.) 

 The most essential criterion of this species lies in the stem, espe- 

 cially in the root of the stem of the hooks of the second series, 

 which presents on its posterior surface a small lunate notch, and 

 also in the tuberculiform curvature on the same side of the stems 

 of the first series. The size of the hooks will be found in the 

 table of hooks. 



From the centre of the hook a small hood- or cap-like ros- 

 tellum, round the base of which fche sac stands, often projects. 

 It is perfectly clear, destitute of pigment at its apex, and has no 

 calcareous corpuscles. 



The sucking discs (ventousen) are nearly circular, frequently 

 somewhat oval, and surrounded by a circular collateral branch of 

 the longitudinal canals, by which a sort of large vascular net is 

 formed. By maceration the sucking discs may be procured, 

 isolated in the form of round balls or discs, in which no perfora- 

 tion is perceptible. By the inversion of these discs in the form 

 of a cup, or in the manner of the finger of a glove, towards the 

 middle line of the animal, they are converted into effective 

 suckers. 



Close behind the sucking discs, and near the head, the vessels 

 collect into four main branches, which are united quite at the front 

 of the head by a common transverse branch, as thick as, or thicker 

 than themselves. Oscar Schmidt has very recently expressed the 

 opinion that anastomoses are effected between the vessels of the 

 two sides, in the fully developed segments (proglottides), by 

 means of transverse branches. 



In the vessels of various Cestoidea, Virchow, Wagener, and 

 others have seen with the microscope a sort of ciliary epithelium 

 projecting into the vessels, and moving in the fluid. It appears then, 

 as I have convinced myself, as if we saw a short luminous flash 

 pass across the field of vision. To recognise this, it is best to 

 work alternately with direct and transmitted light. In many 

 Teenies, but especially in T. solium, I have thought, when employ- 

 ing the highest magnifying powers, that I saw, in the interior of 

 the oscillating contents of the vessels, very small, dark, molecular 

 corpuscles, the nature of which I do not venture to explain. 

 Might not these be pigment molecules, which are deposited from 

 the vessels in the head ? 



