1883.] Contributions to the Anatomy of the Hirudinea. 355 



Colourless amoeboid cells certainly occur in large numbers. In 

 Nejplielis and Trocheta these are almost as large as in Pontobdella. 



In Hirudo and Aulostoma these, although not so large, undoubtedly 

 exist in large numbers, in addition to free nuclei. 



The blood in all the genera coagulates rapidly when withdrawn 

 from the body, filaments of fibrin or some allied substance can be 

 seen forming on the slide. 



Blood Spaces. — These belong to two different systems, which are, 

 however, in direct communication, there being only one fluid. The 

 author shows that one system represents the closed vascular system, 

 while the other represents coelom, vessels, and sinuses. 



The vessels may, to a certain extent, be distinguished by their 

 muscular w r alls, the walls of the sinuses not being muscular. 



Communications between these two systems of spaces, vessels and 

 sinuses exist only at certain definite spots, as in the Rbyncobdellidse ; 

 the vascular dilatations at the sides of the body which are most fully 

 developed in Branchellion affording means of communication ; or else 

 the spaces establishing that communication, although very numerous, 

 have a special mode of formation and a special nature, such spaces 

 constitute botryoidal tissue (Gnathobdellidaa) . 



The author describes at length the distribution of the vessels and 

 sinuses. 



The conclusion at which the author has arrived concerning the 

 coelom in the Hirudinea may be thus summed up : — 



The somewhat scanty embryological evidence which exists upon this 

 point favours the view that the coelom developes by a splitting in the 

 mesoblast ; that it is, in fact, that modification of an enterocoele which 

 Professor Huxley has termed a schizoccele. 



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

 remains most fully developed in the Rhyncobdellidaa, it is reduced to a 

 minimum in Nejphelis and Trocheta, being then represented only by the 

 ventral sinus and its immediate branches. 



In the Rhyncobdellidse, at any rate in Clepsine, Pontobdella, and 

 Branchellion, the ccelomic remnants (sinuses) continue to be lined 

 with ccelomic epithelium cells. In many places they form a continuous 

 layer, but generally some of them have become free and are to be seen 

 floating in the blood. These free ccelomic epithelium cells, which are 

 much larger than the ordinary blood corpuscles, are only to be seen in 

 the sinuses ; they are probably too large to pass through the com- 

 municating channels. In the Gnathobdellidas there is no trace of such 

 cells. 



A process has been taking place, which the author proposes to term 

 diacoelosis — a " scattering of the coelom" — connective tissue growths 

 having more or less completely filled it up, the remnants forming the 

 sinus system. Different remnants remain in different genera. 



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