HIRUDINEA. 



517 



anterior part of the intestine is sometimes called the stomach, and 

 the hinder part may be distinguished as the rectum. 



The perivisceral or body-cavity is a coelom, as is shown by its 

 development and its relations to the renal organs; but in AcantJw- 

 Idella alone is it a spacious cavity divided by septa into chambers, as 

 in the Oligochaetes. In all other leeches it is much broken up, and 

 though retaining its relations with some of the organs, e.g. ventral 

 nerve cords, principal blood vessels, etc., it is largely without a peri- 

 visceral character, and has the form of longitudinal canals or sinuses 

 connected by a complicated system of intercommunicating spaces. 

 This system of canals and spaces is commonly called in leech 

 morphology the sinus system ; the arrangement of it varies some- 

 what in the different genera. 



dl 



Owing to the fact that the sinus system contains a fluid closely resembling the 

 fluid in the blood-vessels, it has often been held to be a part of the vascular system. 

 Leydig was the first to recognize the distinction between the two, and in this he 

 was followed by subsequent workers. But although it is now recognized by the 

 majority of workers that 

 the two systems are dis- 

 tinct, it is still generally 

 held that they are con- 

 tinuous through their finer 

 branches. This view is, 

 as we shall see, based on 

 insufficient evidence, and 

 having regard to the state- 

 ments of Oka and Burger, 

 it seems safe to assert as 

 a fact that the two sys- 

 tems are separate, as has 

 been shown to be the case 

 for all other groups in 

 which the matter has been 

 closely investigated. 



In Clepsine (Fig. 419) 

 there are five longitudinal 

 canals : (1) a median sinus, 



si 



which in the region of the 



PIG. 419. Diagram illustrating the arrangement of the 

 coelom or sinus system in the middle part of the body of 

 Clepsine complanata (after Oka), dc, mlr, vc, sc, he trans- 

 verse sinuses connecting the main longitudinal sinuses ; 

 dg dorsal blood vessel ; dl dorsal sinus ; vg ventral blood 

 vessel ; vt ventral sinus ; n nerve cord ; zl intermediate 

 sinus as a network of canals in this part of the body ; 

 si lateral sinus ; hi hypodermal sinuses, 

 gut diverticula, i.e., in the 



middle part of the body, is divided into a dorsal sinus enclosing the dorsal blood 

 vessel, and a ventral having relation to the ventral blood vessel, nerve cord, and 

 some of the reproductive organs (ovaries and vas deferens) ; (2 and 3) a pair of 

 lateral sinuses placed at the two margins of the body ; and (4 and 5) a pair of 

 intermediate sinuses, which, in the region of the nephridia, are broken up into 

 a network of canals. The median sinus is divided up by a few incomplete, but 

 where they occur segmental, septa. These five sinuses communicate with one 

 another by numerous transverse sinuses, and there is a system of hypodermal 



