14 W. K. BROOKS ON THE 



they are seen to be due to the presence of minute branching tubes, which, spreading over 

 the surface of the body and inosculating, divide it up into small polygonal areas. 



No fluid could be seen to circulate in them, but as they appear at about the same 

 time with the larger blood-vessels of the surface of the body, they are probably the 

 indications of a system of capillary vessels. 



The course of the larger blood-vessels on the posterior face of the mantle is shown, 

 at a somewhat later stage, in fig. 17. A large vessel will be seen to enter the mantle 

 on the median line near the dorsal end of the body. This is the pallial artery from the 

 systemic heart. Passing forwards, it divides into three branches ; a pair of large ones, 

 and a median unpaired smaller one. The latter runs forwards, nearly to the lower edge of 

 the mantle, and divides up into a number of smaller branches. The two larger branches 

 diverge, and running out towards the free edge of the mantle, give rise, on their inner 

 edges, to a number of irregular branches, and on their outer edges to a number of nearly 

 parallel trunks, which communicate with a pair of large venous trunks, each of which 

 receives a smaller trunk from the median tract of the mantle, and then, bending around 

 the side of the body, runs inwards to open into the larger vena cava, from which 

 the blood passes into the branchial heart, and is conveyed to the gills. The branchial 

 hearts appear at quite an early stage of development, but the systemic heart is not 

 developed until about the stage shown in fig. 16. During the later stages of development, 

 and in the adult also, the small size of the gills is no doubt compensated for, to a great 

 degree, by the aeration of the blood while it is passing through the system of vessels neai' 

 the exposed surface of the mantle. 



At the stage shown in fig. 15, the siphon has substantially its adult form, and is made 

 up of two lateral chambers, si', which have been formed from the lateral siphon folds, and 

 which open into the mantle-chamber, but have no external openings ; and a single median 

 chamber, si, on the posterior surface of the body, which has been formed by the union 

 of the two inner siphon folds, and which opens into the mantle-chamber as well as 

 externally. 



At the point where the lateral chambers meet the median chamber, the wall of the 

 siphon is united to the wall of the body, and the three chambers are thus shut off from 

 communication with each other. 



The animal is so perfectly transparent that the valve-like action of the two outer 

 chambers can be perfectly seen, as their free inner edges are thrown out against the 

 mantle so as to close it at each contraction, and the water, which passes in around the 

 whole free edge of the mantle, is thus concentrated in the funnel-shaped middle chamber 

 of the siphon. 



At about this time the valve of the siphon, figure 15, v, is developed as a single 

 unpaired flap, which arises- from the posterior surface of the neck. 



Considerable change has now taken place in the shape of that portion of the yolk 

 which is contained in the head. It is reduced to a long narrow tube, y", which connects 

 the portions contained in the body proper, y'", y"" , with the external yolk sac, y' . 

 The pulsatile space, %, between the outer wall and the surface of the yolk sac, is more 

 plainly shown in this figure than in the preceding ones, although a profile view shows it 

 with equal distinctness at earlier stages. 



