CONTRIBUTIONS TO THE ANATOMY OF THE HIRUDINEA. 439 



5. An external epithelium. This we have seen to be merely 

 the general epidermis of the body, continued into the pro- 

 boscidial wall. 



We have thus very similar arrangements in the two groups^ 

 a pharyngeal epithelium and two layers of pharyngeal muscles ; 

 and while in the Gnathobdellidse the layers external to these 

 are well developed, in the specialised proboscis, they have 

 become reduced, the external circular muscles being absent 

 altogether. 



The genera of Rhyncobdellidae present slight but charac- 

 teristic differences in the structure of the proboscis, which may 

 be seen on comparing figs. 16 — 24. 



In the oesophagus and stomach there are fibres in the wall 

 presenting a not very regular arrangement, but tending to 

 form longitudinal and circular layers. These fibres may be 

 replaced as in Hirudo by branched muscular cells. Such 

 cells are described by Ley dig ^ as occurring also in Piscicola. 

 Somewhat similar cells are also found in the wall of the 

 alimentary canal in Insects and other Arthropods.^ 



Muscles developed in theWalls of Blood-ves sels. — 

 Certain of the blood spaces in the Hirudinea present muscular 

 walls ; ^ these spaces are such as I shall show below represent 

 the closed vascular system of Chsetopoda. 



For a description of the arrangement of these fibres in the 

 various vessels see Circulatory System. 



Muscles developed in connection with the Genera- 

 tive Organs. — Certain muscular developments found in con- 

 nection with the penis and vagina^ as well as in the walls of 

 the ovisac, are better described in connection with those 

 organs. 



Muscles in the walls of the vesicle of the nephri- 

 dium require no special description. 



Muscles developed in the Skin. — Of these I have 

 spoken above. 



1 ' Zeit. f. w. Zool.,' lSi9, p. U. 

 ^ Leuckart, ' Zootomie,' p. 303. 

 ' Leydig, ' Histologic' 



