492 EDWIN S. GOODRICH. 



however, could not trace the dorsal sinus to a ccelomic origin, 

 and since its branches bear the same relation to the cutaneous 

 plexus as those of the latero-abdominal vessels, I am inclined to 

 think that the dorsal sinus may represent the dorsal vessel of 

 other annelids. In that case the ccelomic cavities do not 

 persist dorsally, or have never reached the median dorsal 

 region in the Gnathobdellidse. 



The annular channels may possibly represent the annular 

 ccelomic lacunse so well described and figured by Oka in 

 Clepsine (10), and it may perhaps be through them that the 

 chief communication between the coelom and the vascular 

 system has been established. The observation of the some- 

 what variable relations of these annular channels tends to 

 support this view. 



With the very imperfect knowledge of the development of 

 the coelom and blood-vessels in Hirudo at our disposal, we 

 cannot say for certain at present where the one ends and the 

 other begins, nor whether a given capillary really belongs to 

 the one or the other. Nor can we safely conjecture how the 

 continuity has actually taken place. But one thing seems 

 fairly certain, namely, that it is not only by means of the 

 botryoidal channels that the communication has been brought 

 about. 



It is very tempting to compare the leech with the Verte- 

 brate, in which a third system of spaces — the lymphatic 

 system — has been interpolated, allowing a communication to 

 take place between the originally distinct coelom and blood- 

 vascular system.^ But the botryoidal tissue is not so inter- 



* Tlie structural analogy between the lymphatic system of the Vertebrate 

 aud the botryoidal tissue of Hirudo is in some respects very close. The former 

 develops as an independent set of cavities in the mesoblast, which subse- 

 quently open into the veins and the coelom (Balfour, ' Comparative Embryo- 

 logy '). The latter, according to Biirger (2), develops also as a number of 

 independent channels, hollowed out in strings of cells of n.esoblastic and even 

 peritoneal origin, which later come into connection with the lisemolymph 

 system. The functions of the lymphatic and botryoidal systems must be 

 quite different, since the latter is not in any way specially related to the 

 alimentary canal. 



