454 ALFRED GIBBS BOURNE. 



Valuable as the theories above mentioned were, and minute 

 and detailed as are the descriptions of the vascular system 

 by Budge^ in Clepsine, by Gratiolet" in Hirudo, by Bidder, 

 in Nephelis^, and by Vaillant* in Pontobdella, it is only 

 by the use of the most recent histological methods, and in the 

 light of recent theories concerning coelomic spaces and blood 

 sinuses, that I have been able to form a consistent scheme of 

 the vascular spaces in the group as a whole (see p. 474). 



Leydig's division of the vascular spaces into a contractile 

 and a non-contractile system, appears to hold good, not only, 

 for the four genera specially investigated by him, but, for the 

 whole group, and as De Quatrefages pointed out, the former 

 seems to represent the closed vascular system of Chsetopoda, 

 while the latter is probably coelom, whether in the state of 

 new formation, or, gradually becoming occluded, i. e. in a state 

 of degeneration. My object has been to determine throughout 

 the group, what spaces are to be regarded as belonging to one 

 system and what to the other. The whole system being in abso- 

 lute continuity, it becomes work of considerable difficulty to 

 determine whether any given space or system of spaces belongs 

 to one system or to the other. The presence or absence of 

 rhythmic contractility only possesses importance for the solu- 

 tion of this question in the case of the larger trunks. 



The existence of various organs, alimentary canal, nerve- 

 cord, testis, &c., in certain spaces, points quite conclusively to 

 the coelomic nature of such spaces. The funnels of the nephridia 

 play a very important role in this respect, occurring in dif- 

 ferent spaces in different genera. By the word " ccelom " I 

 understand a space or set of spaces excavated in the mesoblast 

 and distinct from blood-vessels, such as is the body-cavity of 

 Chsetopods and of Vertebrates, and I do not undertake in any 

 way to discuss whether such space is a pseudocoel or an enterocoel 



' Verliand. Nat. Hist. Vereins. der Preuss. Rheinlande, 1849. 

 2 'Ann. sci. nat. Zool.,' 18G2. 



^ ' Untersuchungen iiber dass Bliittgefassystem einiger Hirudineen,' 

 Dorpat, 18G8. 

 ^ Ann. sci. nat. Zool.,' 1870. 



