376 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



they are seen to be due to the presence of minute branch- 

 ing tubes, which, spreading over the surface of the body 

 and inosculating, divide it up into small polygonal 

 areas. 



No fluid can 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. 201. 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 

 forward, 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. 201. During the later 

 stages of dev^elopment, and in the adult also, the small 

 size of the gills is no doubt compensated, to a great de- 

 gree, by the aeration of the ])lood while it is passing 

 through the system of vessels near the ex})osed surface of 

 the mantle. 



