In the completely developed state the echinococcus-heads exhibit somewhat variable characters as to size and form, the latter differences being, for the most part, dependent upon their degree of contraction and vitality. In the perfect condition they vary from the 1/60″ to the 1/100″ in diameter, being usually about the 1/80″. They are solid, and when stretched out exhibit an hour-glass-like constriction at the centre of the body, which divides the scolex into an anterior part supporting the rostellum and suckers, and a posterior part which has been compared to the caudal vesicle of ordinary Cysticerci. The rostellum supports a double crown of hooks, but the disparity of the two series is scarcely sufficiently marked to render their distinction obvious. The hooks of the smaller row vary in size from 1/1040″ to 1/830″ of an inch, whilst those of the larger series are from 1/830 to 1/555″. In all instances the root-processes are incompletely developed, and consequently vary in thickness. They are, as Leuckart also has stated, apt to exhibit abnormalities. [Illustration: FIG. 33.--Separate _scolex_, or echinococcus-head. Magnified 500 diameters. After Huxley.] In regard to the development of the echinococcus-heads it further remains for me to observe that a distinct water-vascular system is recognisable in the scolices. By the intervention of the pedicle of the scolex this system is connected with the brood-capsule, and also with the vessels of the maternal endocyst. In the scolex there exists a circular channel immediately below the rostellum, and this ring, on either side, gives off two vessels which pass downwards in a tortuous manner, internally, until they arrive at the pedicle where they unite to form two channels, which latter are continued into the vascular system of the maternal endocyst. In the retracted condition their position, of course, becomes very much altered, and they form loops on either side of the central line which marks the space leading down to the inverted head. Neither Prof. Huxley nor myself have seen these vessels, which Leuckart observed in the scolex itself, but Huxley discerned some apparently loose cilia in the granular parenchyma of the body; their longitudinal measurement being about the 1/3500 of an inch. [Illustration: FIG. 34.--An Echinococcus brood-capsule (flattened by pressure). Magnified about 120 diameters. From a drawing by Professor Busk.]