16 LEECHES 



(1862): the latero-dorsals, latero-ventrals and latero-laterals. 

 These break up into capillaries in the botryoidal tissue and the 

 body wall, but the capillaries unite again to form tributaries of the 

 dorsal and ventral sinuses. The detailed course of the circulation 

 has not been worked out, but since blood is driven forward by the 

 contraction of the lateral sinuses, there is probably a compensatory 

 backward flow in the dorsal and ventral sinuses. 



The blood consists of a plasma, coloured red by haemoglobin in 

 solution, and containing numerous amoeboid corpuscles together 

 with some chloragogenous cells. 



8. Histology 



Under this heading are described some of the details which may 

 be seen in transverse sections. 



Epidermis 



On the outer body surface is a very thin cuticle, secreted by the 

 epidermal cells and renewed at intervals of a few days. The epi- 

 dermal cells are columnar and widen at the outer ends to form 

 pentagonal heads which fit closely against the heads of neigh- 

 bouring cells. The inner ends are cylindrical and there are spaces 

 between the cells into which penetrate blood capillaries, nerve 

 endings, pigment cells and dermal fibres. Derived from the epi- 

 dermis are various kinds of unicellular glands. On the general body 

 surface there are two kinds of m-ucus glands : pear-shaped glands 

 with the body of the cell just under the epidermis and a narrow 

 neck opening at the surface, and elongated tubular glands which 

 penetrate down into the muscle layers (Fig. 9). In the anterior 

 head region all the available space between muscles is occupied by 

 densely packed pear-shaped glandular cells whose secretion is 

 poured on to the surface of the anterior sucker. The posterior 

 sucker is similarly equipped. The salivary glands, which lie 

 between the pharynx and the body wall, are modified epidermal 

 cells. They are unicellular pyriform glands with long ductules 

 leading to the jaws. Like the general body epidermis, the epidermis 

 of the jaws secretes a cuticle and bears the apertures of these 

 modified epidermal glands. In the clitellar region there are, in 

 addition to the mucus glands, two types of gland concerned with 



