4G0 ALFRED GIBBS BOURNE. 



such as it is, consists of nothing but these cells ; there is no 

 endothelial lining occupying the interior of this capsule ; there 

 are an immense number of branched connective-tissue cells, 

 whose branches unite, forming a complete reticulum (loc. cit., 

 p.) reminding one most strongly of the medullary substance of 

 the lymphatic gland of a Vertebrate; it is possibly a primitive 

 lymphoid tissue. 



Between the meshes of this network are packed numerous 

 white blood-corpuscles (loc. cit., q.) . (The majority of these 

 have been omitted from the figure to show the reticulum.) 



We have here morphologically the simplest possible lymphatic 

 glands (more strictly comparable to the lymphatic nodules 

 which occur within or around lymphatics, or in other parts of the 

 body, these nodules lying in what may be termed the lymphatic 

 space) ; and I think it highly probable that these organs play 

 also the physiological role of the lymphatic gland producing 

 the white blood-corpuscles. 



The exact mode of origin and relations of these might throw 

 light upon lymphatic glands of Vertebrata. These struc- 

 tures in Pontobdella seem to be only special results of that 

 process of diacoelosis which is going on throughout the 

 group — ingrowth of connective tissue " scattering " or carry- 

 ing with it coelomic space. 



Clepsine. 



The system of blood spaces in Clepsine has formed the subject 

 of a large number of special memoirs by Filippi, O. F. Miiller, 

 Grube, Leydig, Budge, Leuckart, Bidder and Whitman. 



The most recent author, Whitman,^ while citing a large 

 number of the older authors, curiously enough has not seen 

 the most complete description, a description in many respects 

 more complete than his own, that by Budge.- 



I propose to deal with the descriptions by Leydig, Budge, 

 and Whitman. 



1 'Quart. Journ. Micr. Sci.,' 1878, p. 286. 



^ Verb. Nut. Veieius. der Preuss, Rheiulaude und Westplmlens, 1849. 



