328 AUSTRALIAN HIRUDINEA, I., 



becomes attached to the wall of the pharyngeal sac which 

 surrounds the whole of the proboscis except a small part at the 

 posterior extremity, occupying about two annuli. 



The wall of the pharyngeal sac consists of a layer of flattened 

 epithelial cells which are found to be continuous with the 

 epidermis of the bod}^ It is quite devoid of the glandular 

 elements which occur as modified epidermal cells in the body- 

 wall. 



The wall of the proboscis comprises five distinct elements : — 



(1) Pharyngeal epithelium, consisting of a single layer of minute 

 flattened cells. 



(2) Radial muscles, occupying a great portion of the pharyngeal 

 wall which passes out between the circular muscle elements, and 

 then becoming dilated to abut on the epithelium of the external 

 surface of the proboscis. 



(3) Circular muscles, forming three crescentic masses lying, 

 together with the radial muscles, in the triangular areas formed 

 by the contour of the inner wall of the proboscis. 



(4) Longitudinal muscles, occurring as bundles scattered among 

 the outer portions of the radial muscles, which occupy the greater 

 part of the region in which the longitudinal muscles occur. 



(5) External epithelium, consisting of flattened cells, continuous 

 at the posterior end of the pharyngeal sac with its wall, and so 

 representing portion of a modified epidermis. 



The proboscis leads posteriorly into the oesophagus. When the 

 proboscis is retracted, the oesophagus appears as a tube running 

 parallel and dorsally to it, and extending through about eleven 

 annuli. Its wall is very much folded, and quite different in 

 character from that of the proboscis and crop. The internal 

 lining la}^er consists of cells which take a vivid blue stain with 

 hematoxylin. This layer is very much folded, reminding one of 

 the general appearance of the pharyngeal sac when the proboscis 

 is retracted. The folding of the layer is due to the contraction 

 of the oesophageal muscles when the proboscis is withdrawn. 

 The muscles of the wall of this portion of the digestive system 

 consist of circular muscle fibres immediately external to the 



