476 ALFRED GIBBS BOURNE. 



nature of a cfecal diverticulum formed as a special development 

 of botryoidal tissue. The lumen in both genera is in direct 

 communication with the ventral sinus through the intervention 

 of botryoidal tissue, and with the lateral vessels of its own side 

 through the intervention of the branches of the latter, which 

 terminating as capillaries, open into the botryoidal tissue. 



General conclusions with regard to the coelom in 

 the Hirudinea. 



The somewhat scanty embryological evidence which we 

 possess upon this point favours the view that the coelom deve- 

 lops by a splitting in the mesoblast, that it is in fact that 

 modification of an enterocoele which Huxley has termed a 

 schizocoele. 



This cavity persists to some extent in all the genera, and 

 while it remains most fully developed in the Rhynchobdellidse, 

 it is reduced to aminimum in Nephelis and Trocheta, being 

 there represented only by the ventral sinus and its immediate 

 branches. 



In the Rhynchobdellidse, at any rate in Clepsine, Ponto- 

 bdella and Branchellion, the ccelomic remnants (sinuses) 

 continue to be lined with coelomic epithelium cells. These cells 

 (figs. 41 — 43) are exactly similar in all three genera (I have 

 not looked for them in Piscicola). In many places they 

 form a continuous layer, but most generally some of these 

 have come free, sometimes almost all, and are to be seen float- 

 ing in the blood. These free coelomic-epithelium cells are only 

 to be seen in the sinuses, they are probably too large to pass 

 through the communicating channels. In the Gnathobdellidae 

 there is no trace of such cells. 



A process has been taking place which I propose to term 

 diaccelosis, a ''scattering'^ of the coelom, connective-tissue 

 growths having more or less completely filled it up, the rem- 

 nants forming the sinus system. 



Different remnants remain in different genera. The organs 

 which in animals possessing a well-developed coelom lie within 

 that coelom, either get blocked out by connective-tissue growth, 

 or remain enclosed in the remnants. The same organs may 



