74 A FURTHER KNOWLEDGE OF THE CYSTIC CESTODES, 



twenty in number, and arranged in a single row. Each hook 

 (Fig. 13) has a total length of 03 mm. and is i)rovided with an 

 internal root ("dent," " Hypomochlion," or "talon") and an 

 anterior root (Leuckart's "posterior process," Villot's "manche"). 

 The anterior root, which is inserted into the tissue of the ros- 

 tellum, runs forwards in a line with the free part of the hook 

 (Villot's "lame"), while the internal root runs backwards almost 

 parallel with the free part. The free part of the hook is '018 mm. 

 long, the internal root -0125 mm., and the anterior root -012 mm. 



The great development of the internal roots of the hooks of 

 Monocercus Didymogastris may be taken as their most striking 

 characteristic, and that they differ most markedly in form from 

 those of Monocercus arionis and M. gJomeridis a comparison of 

 Fig. 13 with Villot's figures* will clearly show. 



The posterior part of the head, behind the posterior end of the 

 rostellum and the suckers, may be taken to represent the neck. 

 This neck region becomes continuous with a thin layer of deeply 

 staining small-celled tissue, which passes forwards, completely 

 enclosing the head. This represents the second part of the Cysti- 

 cercoid — the body. Anteriorly it becomes continuous at the 

 aperture of evagination with the outer wall of the Cysticercoid. 

 The outer wall represents the third part of the Cysticercoid — the 

 caudal bladder. In sections it is to be clearly distinguished from 

 the layer forming the body. It is composed of a larger-celled 

 tissue and does not stain so deeply with hsematoxylin, and to it 

 the calcareous bodies, distributed throughout its thickness, are 

 almost entirely confined. Externally it is covered by cuticle, 

 which, at the aperture of evagination, turns in, becoming con- 

 tinuous with what will, in the evaginated Cysticercoid, be the 

 external cuticle of the body. Both in its histological structure 

 and in its relations to the other parts of the Cysticercoid, this 

 outer wall clearly corresponds to the caudal bladder of the ordi- 

 nary Cysticerci. 



* Loc. cit. pi. XII. figs. 10 and 11. 



