EXISTENCE BETWEEN BODY-CAVITY AND VASCULAR SYSTEM. 23 



name archi-coelom for this system of spaces, and in which, as is 

 stated above, the inner ends of the nephridia sometimes lie. 



I have already drawa attention' in a previous paper to the 

 fact that the cavity of the heart in the embryo Lamprey is 

 continuous with the segmentation cavit}^ In my account of 

 the development of the heart the following passage occurs : 

 "From the fact... that the mesoblast behind the heart has not 

 split into somatic and splanchnic layers, and not united ventrally, 

 it will be seen that the cavity of the heart communicates posteriorly 

 with the space between the ventral yolk cells (hypoblast) and the 

 epidermis. Such a space would be equivalent to the segmentation 

 cavity." Such a space exists, and becomes for a time crowded 

 with blood corpuscles budded off from the free edges of the 

 mesoblast, which occupies its dorso-lateral angles. These sub- 

 sequently become enclosed in a secondary cavity formed by the 

 down-growth and fusion of the mesoblastic laminae, and so come 

 to lie in the heart and subintestinal veins. 



When I wrote the above I was not aware that Biitschli" had 

 conjectured that the cavity of the vascular system of Vertebrates 

 was derived from the segmentation cavity. What he conceived 

 from theoretical grounds I was able to see in the developing Lam- 

 prey. I think we are therefore justified in applying to the vascular 

 system of Vertebrates the term archi-coelom, which, Hubrecht has 

 suggested for the blood-containing spaces in the Nemertea. 



The system of spaces then of Nemertea which contain blood, 

 and in which the inner ends of the nephridia sometimes lie, are 

 not coelomic in their nature, but archicoelomic ; and as the cavity- 

 sheath of the proboscis has a similar origin, we are driven to the 

 conclusion that there is no coelom in these animals, and therefore 

 there can be no communication between the coelom and the vas- 

 cular systems in this group, such as has been demonstrated for the 

 Hinidinea. 



The Gephyrea form another group of animals in which, like 



1 "On some points in the Development of Petromyzon fluviatilis." Q. J. M. S. 

 Vol. XXVII. p. 325. 



2 " Ueber eine Hypothese beziiglich der phylogenetischen Herleitung dea 

 Blutgefassapparates einea Theils der Metazoen." Morph. Jahrbuch, Vol. 8, 1883. 



