1883.] Contributions to the Anatomy of the Hirudinea. 351 



In Hirudo the external evidences of metameric segmentation are not 

 so pronounced, but minute examination shows that point for point 

 Hirudo agrees almost absolutely with Pontobdella. The somite com- 

 prises five annuli, the clitellum three somites, the generative pores 

 being placed in the two more posterior. 



There is an absolute regularity in the position of the nephridal 

 pores, these occurring in the posterior annulus of a somite. 



The internal organisation bears the same relation to these external 

 characters which was stated to obtain in Pontobdella. The similar 

 characters of other genera are less fully dealt with. 



The external characters thus express in the fullest manner the 

 metanierically segmented character of the internal organisation, and 

 these relations are identical throughout the group. 



Shin. 



The skin consists of — 



1. Cuticle. 



2. Epidermis. 



3. Dermis. 



Cuticle. — This presents similar characters throughout the group. 



Epidermis. — This consists throughout the group of a single layer of 

 nucleated cells. These vary in size in different genera. 



Two varieties of connective tissue may intrude upon the series of 

 epidermic cells : — 



1. Pigmented connective tissue cells, and 



2. Capillaries of the vascular system. 



No pigment is ever developed in the epidermic cells themselves, as 

 is the case in Peripatus (Balfour). The extent to which this intrusion 

 takes place (in respect of the pigmented tissue at any rate) varies 

 much in genera and species, and even in individuals. In most leeches 

 it varies also from point to point, producing the coloured pattern upon 

 the surface of the body. 



The intrusion of capillaries only takes place in the Gnathobdellidee ; 

 in the Rhyncobdellidae they stop short of the epidermic series of cells 

 or merely insert themselves between the bases of these cells. 



Two modifications of epidermic cells may take place — * 



1. They may become glandular. 



2. They may become sensory. 



1. Epidermic Glands. — Two kinds of epidermic glands are to be 

 distinguished. 



i. Mucous glands. — These remain dermic in position and occur all 

 over the surface of the body. They may remain small, and in the 

 series of epidermic cells not passing below them (Piscicola), or 



* The author takes no account here of the origin of the nephridia ; they may be, 

 however, glandular modifications of epidermic cells, but if so are much specialised. 



