Structure of an Australian Land Leech. 227 



The dorsal sinus lies close to, and above the alimentary canal. 

 It is a definite structure consisting of a thin epithelial lining 

 surrounded by connective tissue, but possessing no muscle fibres 

 in its walls. With regard to its branches the dorsal sinus may 

 be said to be "segmental," that is the paired branches are at any 

 late in the middle region, regularly repeated in the different 

 segments, each pair corresponding to and I'eturning blood from 

 each pair of diverticula of the alimentary canal. At the extreme 

 anterior end the dorsal sinus still remains a single channel 

 disappearing in its smaller branches only at about the level of 

 the second pair of eyes. 



Between each branch from the diverticula is one bringing back 

 blood from a nephridium. This arrangement continues back as 

 far as the eleventh diverticula. At this point there is the usual 

 pair of vessels from the alimentary canal walls and also the pair 

 from the thirteenth nephridia entering the dorsal sinus at the 

 level of the stomach. 



A small branch joins the sinus, returning blood from the 

 ventral and lateral walls of the stomach. Vessels from the 

 fourteenth, fifteenth, sixteenth, and seventeenth nephridia pour 

 their contents into the sinus. Posterior to this is a pair of 

 vessels which collects blood from the intestinal wall by numerous 

 branches. These two vessels unite and form the extreme 

 posterior portion of the sinus. 



The ventral sinus is situated immediately within the body wall 

 in the mid-ventral line. The structure of this tube is similar 

 to that of the dorsal sinus. Within it lies the nerve cord. 



In each segment, as in the case of the dorsal sinus, vessels open 

 into the ventral sinus, bringing blood from the organs and tissues 

 of the animal, and from the skin of the body wall where it is 

 aerated. The lateral vessels are found just beneath the body 

 wall one on each side of tlie leech. They are larger than the 

 sinuses, and are more definite in structure (Figs. 20, 21, 22, bv.) 



The wall consists of large circular muscles (Fig. 22, cm.) which 

 have the characteristic inner granular part, and the outer 

 cortical layer more solid and definite. Within these muscles 

 is a layer of longitudinal fibres (Fig. 22, LM.), muscular in 

 appearance thougli very small and taking the stain faintly. 

 These vessels owing to their muscular character play an 

 important part in the circulation of the blood. 



