4.42 A. A. W. HUBRECHT. 
intestine is wholly filled up by the connective tissue, which 
there obtains a hyaline, transparent, and gelatinous character, 
with cells sparsely interstrewn, and with variously-shaped 
fibres. 
The spaces belonging to the blood-vascular system of Lineus, 
viz. the lacune round the cesophagus and in the head, and the 
three longitudinal vessels with metameric anastomoses in the 
rest of the body, arise merely by the fact of the connective 
tissue not obliterating these spaces mentioned, but there 
affecting the shape (1) of an endothelium lining the cavities, 
(2) of a basal membrane, (3) of an outer coating in which 
separate fibres may sometimes be distinguished. The direct 
passage of the archicelom into the blood-space can be very 
demonstratively studied in the cesophageal region, and also 
for the dorsal vessel beneath the proboscidian sheath (fig. 14). 
Elsewhere this has been figured more in detail (1. c. (80), 
pl. iv, figs. 64—67; pl. v, figs. 72—83 and 87). 
It may here be added that the three longitudinal vessels 
cannot be easily observed during all the early larval stages, 
principally because of the close application of the hypoblast 
against the body wall. 
However, when we have only the choice between two possi- 
bilities: (1) that the blood-vascular system, together with the 
lacuna round the cesophagus and the blood-vessel inside the 
proboscidian sheath, arise by a splitting process ad hoc in the 
mesoblast ; or (2) that, together with the proboscidian cavity, 
inside which the dorsal vessel is partially enclosed, it repre- 
sents the last remnants of the archicel, i.e. of that cavity in 
which already in the blastula stage a fluid was contained, and 
a movement of this fluid was possible, there can be no 
doubt which of the two is the more simple and more natural 
explanation. The blood-vascular apparatus as well as the pro- 
boscidian sheath are thus of very primitive significance. A 
fact of some importance is this, that in the adult the posterior 
brain-lobes are bathed by the circumcesophageal blood-lacuna. 
They accordingly remain situated, notwithstanding their coales- 
cence with the anterior brain-lobes and their external openings, 
