218 Mr A. E. Shipley, On the Existence of Communications [May 7, 
continuous with the segmentation cavity. 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 
Lamprey. I think we are therefore justified in applying to the 
vascular system of Vertebrates the term archi-coelom, which Hu- 
brecht has suggested for the blood-containing spaces in the Ne- 
mertea. 
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 
Hirudinea. 
The Gephyrea form another group of animals in which, like 
the Hirudinea, there is direct communication between the coelom 
and the blood vessels. 
The body-cavity of Sipunculus is developed as a split im the 
mesoblastic bands; the cells lining it give rise to the generative 
products; and the nephridia open at their internal ends into it. 
The blood vascular system arises late. Hatschek? describes its 
first origin during the metamorphosis of the larva, lying on the 
dorsal side of the alimentary canal. Although his description is 
not very detailed, there is nothing to shew that we have here to 
do with anything more than a normal blood vessel. 
In the adult the main longitudinal vessel lies well surrounded 
1 “Ueber eine Hypothese beziiglich der phylogenetischen Herleitung des 
Blutgefassapparates eines Theils der Metazoen.” Morph. Jahrbuch, Vol. 8, 1883. 
? B. Hatschek, “‘ Ueber Entwicklung von Sipunculus nudus.” Arbeiten aus 
dem Zoologischen Institut, Wien, Bd, v. p. 33, 
