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XX. On the Circulation of the Blood in Pegea, as bearing on the question of a Lining to the 
Vascular System in the Tunicata m general. By John D. Macdonald, R.N., F.R.S. 
Communicated by Geokge Busk, Esq., F.R.S., Sec. Z.S. 
Read April 18th, 1861. 
In a genus of Salpians which I have identified with the Pegea * of Savigny, stellate 
pigment-cells are distributed equally over the parietes of a very perfect vascular system, 
and are nowhere else to be found. Now, it is natural to conclude that these cells must 
rest upon the basement membrane, or at least on an extension of some delicate structure 
that may perhaps be worthy of the name of a vascular lining. In this case the pigment- 
cells seem to be connected with the inner surface of the canals ; for, as dissolution ap- 
proaches, their caudate processes contract, and they assume the appearance of dark dots, 
O 
which gradually fall off and commingle with the blood-corpuscles. This circumstance 
perhaps rather favourable to the opposite doctrine, but an inspection of the accompanyii 
figures illustrating the scheme of circulation in Pegea, will show that some more satisfac- 
tory explanation of actual appearances is required, at least in the example given. In the 
numerous specimens examined, I could discover no irregular blood-sinuses or lacuna, 
with the exception of the perivisceral chamber, and the canals of the " elaobhist" com- 
municating with it ; and in fact, though the course of the blood, as in other Ascidians, is 
sometimes in one direction and sometimes in the other, in every part, it is through cir- 
cumscribed and definite canals with distinct lines of contour. 
The heart in Pegea (Plate XXXV. fig. 1, d), as in other Salpians, is semicylindrical and 
ed. Prom its fore part two long trunks (f& g) take their 
One of 
passes forwards above or to the left side of the endostyle (/;), while the other (/) curves a 
little backwards and to the right side, and after having communicated with the perivis- 
ceral sinus (a), doubles over the posterior extremity of the endostyle, below and to the 
right of which it runs forwards. 
The posterior extremity of the heart communicates directly with the perivisceral sinus, 
and this latter is connected with the canals of the elceoblast (c) above, while two wide 
blood-channels are given off from it posteriorly. One of these divides into a principal 
and several secondary respiratory vessels (1), all of which inosculate freely through an 
elaborate plexus of capillaries filling up the body of the gill band (*). The other trunk 
(**) runs backwards in the median line for some little distance, and then divides into two 
branches, which pass downwards and backwards, subdividing in the walls of the body. 
A star-like sinus, reminding one of the torcular KeropMU of the human dura mater, 
The species fisrured 
' me species hgured (riate xxxi. ng. 3) m me aims ui ucbn*kuj *«" »^— — ~~ r - 
observed by me, a gemmiparous member of the same species of which Savigny only figures the oviparous form- 
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VOL. XXIII. 
