[ 871 J 



XX. On the Circulation of the Blood in Pegea, as bearing on the qtiestion of a Lining to the 

 Vascular System in the Tunicata in general. By John D. Macdonald, R,N,, F.B.S. 

 Communicated by George Busk, JEsq., F.B.S. , Sec. B.S. 



Read April 18th, 1861. 



In a genus of Salpiaus which I have identified with the Pegea '^ of Savigny, stellate 

 pigmtnt-cells are distrihuted equally over the parietes of a very perfect vascular system, 

 and are nowhere else to he 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 he 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, 

 which gradually fall off and commingle v, ith the blood-corpuscles. This circumstance is 

 perhaps rather favourable to the opposite doctrine, but an inspection of the accompanying 

 figures illustrating the scheme of circulation in Pegea, will shoAv 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 lacnncB, 

 with the exception of the perivisceral chamber, and the canals of the ^ elceoblasf com- 

 municating with it ; and in fact, though the course of the blood, as m 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 scmicylindrical and 

 curved. Erom its fore part two long trunks (/'& g) take their rise. One of these (g) 

 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 («), doubles over the posterior extremity of the endostyle, below and to the 

 right of which it runs forwards. 



The posterior extremity of the heart comumnicates directly with the perivisceral sinus, 

 and this latter is connected with the canals of the elxoblast (c) above, while two Avide 

 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 (e). The other trunk 

 (j) 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 Ilerophili of the human dura mater, 



* The species figured (Plate xxxi. fig. 3) in the Atlas of Lesueur and Petit as Salpa vivipara is obviously like that 

 observed by me, a gemmiparous member of the same species of vfhich Savigny only figures the oviparous form. 

 VOL. XXIII. 3 E 



