THE ENTEROPNEUSTA. 61 



and it is in communication with the body cavity by means of 

 the interstices between the cells bounding its anterior end. 



Its further development is involved with that of the heart 

 which had better be now described. The heart arises in ani- 

 mals with three pairs of gill-slitSj as a horizontal split in the 

 tissue between the notochord and the sac of the proboscis 

 gland. Its walls are very thin [v. fig. 52). From the first it 

 appears to contain blood, which is apparently non-corpusculated, 

 and can be coagulated by reagents. Whether the heart is 

 originally in connection with the dorsal vessel or not could 

 not be determined. Its walls soon become slightly muscular 

 [v. figs. &7 and 97), and the pulsations, which can be dimly 

 discerned through the skin in the living state, are doubtless 

 occurring in this vesicle. 



After the formation of the heart a plexus of vessels in con- 

 nection with it is formed among the mesoblastic cells covering 

 the tip of the notochord (fig. 50). As this occurs the cells 

 standing on the capillaries assume a pyriform shape, the sharp 

 ends being fixed to the vessels and the wide ends free. These 

 wide ends acquire a very transparent appearance, as though 

 filled with fluid (fig. 49). These bunches of capillaries eventu- 

 ally acquire a great development and communicate with two 

 larger blood-vessels (fig. 53, b. v.), and with a sinus in the 

 periphery of the gland. 



The sac of the proboscis gland anteriorly becomes filled up 

 with a quantity of loose tissue, in which some granules of a 

 yellowish colour are embedded. 



In B. minutus these yellow granules are of much com- 

 moner occurrence (v. .fig. 98). The capillaries of the gland are 

 more regularly arranged. 



In B. salmoneus the capillaries are still more regular, 

 running parallel to each other to the periphery of the gland, 

 where they are united in a plexus of larger vessels (cp., figs. 



95 97). The outer cells of the gland are modified to form a 



peculiar tissue (fig. 97). They are large cells, which stain 

 deeply and have a nucleus usually on their outline. The cells 

 standing on the capillaries contain some yellow granules, and 



