452 ALFRED GIBBS BOURNE. 



formed. Usually not more than two or three cells, however, 

 surround the lumen in any particular region. 



This important question leads us so far away from our subject 

 and rests at present upon such a speculative basis that it is not 

 advisable to discuss it further here. 



V. Blood — Blood-Corpuscles. 



There is only one vascular fluid in the Hirudinea which 

 corresponds to both coelomic and red vascular fluids as found in 

 the Chffitopoda. 



Curiously erroneous statements exist with regard to this fluid ; 

 the presence of corpuscles has been universally denied, and a 

 colouring matter has been described where none such exists, 

 and its existence denied in genera where it is always present. 



Bhyncobdellidse. — The blood is colourless. Colourless 

 amoeboid corpuscles occur in very large numbers. A nucleus 

 is present surrounded by a considerable amount of granular 

 amoeboid protoplasm (fig. 39). This protoplasm often becomes 

 drawn out into filamentous processes, which, as is the case with 

 the majority of amoeboid corpuscles, tend while upon the micro- 

 scope-slide to fuse together. 



These corpuscles are probably formed from the walls of the 

 capillary vessels : these in Pontobdella, at any rate, where I have 

 examined them closely (fig. 47), present naked protoplasm upon 

 their inner surface with embedded nuclei. The naked proto- 

 plasm presents amoeboid processes. These corpuscles are 

 possibly also formed in the " lymphoid " nodules described below 

 (fig. 44). In addition to these amoeboid corpuscles there may 

 be constantly seen in Clepsine, floating in the larger blood 

 spaces, much larger rounded cells ; these, as I shall show 

 below, are coelomic epithelium cells which become detached 

 from the walls of the sinuses, while their size prevents them 

 from being carried into the true blood-vessels. Similar cells 

 may be seen in a few places in Pontobdella (figs. 10, coel. ep. ; 

 53, coel. ep.). 



Gnathobdellidse. — The blood is red, the plasma con- 



