£6 THE LEECH. 



3. The inner or longitudinal layer of muscles lies im- 



mediately within the oblique layer, and is by far the 

 thickest of the three layers. 



The fibres run longitudinally, and are therefore 

 seen in transverse section. They are arranged in 

 bundles, and have a very characteristic appearance, 

 looking, for the reason noticed above, like rings of 

 various sizes packed together in groups, and usually 

 more or less polygonal from mutual pressure. Each 

 ring is really a transverse section of a single muscle- 

 cell. 



4. The dorso-ventral 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 

 coelomic systems of higher animals, are in free communica- 

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

 owing to the deeply staining clotted blood with which they are 

 filled. 



1. The dorsal and veatral 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. 



