54 THE LEECH 



more or less polygonal from mutual pressure. Each 

 ring is really a transverse section of a single muscle- 

 ceU. 

 4. The dorso-Tentral muscles are bundles of muscle-fibres 

 crossing the body more or less obliquely from the 

 dorsal to the ventral surface. They pass between 

 the bundles of longitudinal muscles and through the 

 oblique and circular layers, their ends spreading out 

 just beneath the epidermis. 



D. The Alimentary Canal. 



A section across the middle portion of the body will cut the 

 crop, of which the median portion and the lateral diverticula 

 will in most sections appear separate from each other. 



The walls consist of an outer connective-tissue layer with 

 a few muscular fibres, and an inner lining formed of a single 

 layer of short columnar epithelial cells. The folds into which 

 the inner surface is thrown involve the connective-tissue layer 

 as well as the epithelium. 



E. The Blood-vessels and Sinuses. 



These two systems, corresponding to the vascular and 

 ccelomic systems of higher animals, are in free communica- 

 tion with each other. They are readily recognised in sections, 

 owing to the deeply staining blood-clots with which they are 

 filled. 



1. The dorsal and ventral sinuses lie in the median plane, 



above and below the crop respectively. They have 

 thin non-muscular walls, and the ventral one encloses 

 the nerve-chain. 



2. The peri-nephrostomial sinuses are easily recognised 



in sections that pass through the testes, on the dorsal 

 surfaces of which they lie. 



3. The lateral vessels are of large size, and have distinct 



muscular walls. 



4. The capillaries are very thin-walled tubes, the distribu- 



tion of which can be readily followed owing to the 

 contained blood. The intercellular plexuses of the 



