518 



ANNELIDA. 



sinuses communicating with the others. The fluid in the sinuses contains two 

 kinds of corpuscles, the smaller are exactly similar to the corpuscles of the 

 blood, the larger are apparently detached from the epithelium lining the sinuses, 

 and are not found in the blood. That these sinuses are coelornic is shown by 

 the fact that the nephridia open into them, and that they do not communicate 

 with the vascular system. The latter fact, though long unrecognized on account 

 of the difficulty of distinguishing between blood-vessels and the finer ramifi- 

 cations of the sinus system, can hardly be disputed if Oka is correct in his 

 statements that the two can easily be distinguished in stained preparations, 

 and that the corpuscles of the sinus fluid differ as above explained from those 

 of the blood. Moreover, developrffentally the two systems are absolutely 

 distinct. 



l o n vs 



FIG. 420. Diagrams of transverse sections through various leeches to show the relation of the 

 sinus system (coelom) to the other organs. 1, Clepsine. 2, Nephelis. 3, Pontdbdclla. 

 h, Hirudo. (After Bourne, from Perrier). a alimentary canal ; ds dorsal sinus ; dv dorsal 

 vessel ; /nephridial funnel ; c, Is lateral sinus ; Iv lateral vessel ; n nerve cord ; na nephridio- 

 pore ; o ovary ; t testis ; vs ventral sinus ; v ventral vessel. 



In other leeches the coelom is very similar (Fig. 420). In Pontoldella it is 

 even more complete than in Clepsine. In Hirudo the lateral sinuses are absent, 

 and in Nephelis the dorsal. 



Vascular system. The blood is colourless in the Wiynchobdellidae, 

 and coloured red with haemoglobin in the Gnathobdellidae. In all 

 cases it has approximately the same colour as the coelomic fluid.* 

 Since Leydig's work the main blood vessels have been distinguished 



* In the young Nephelis the blood is red and the coelomic fluid yellow 

 (Burger). 



