406 UROCHORDA OR TUNIC ATA. 



the blood is spread out in vessels, and is thus aerated. The 

 peribranchial chamber is lined by ectoderm, for it is formed 

 in development by the union of two ectodermic invagina- 

 tions, which grow towards each other. The first gill slits are 

 formed by the fusion of small diverticula of the pharynx 

 with two separate peribranchial invaginations. In the adult 

 the slits are very numerous and of secondary origin ; they 

 are formed partly by the division of primary slits, partly by 

 new perforations of the wall of the pharynx. With regard 

 to the development of all these structures, however, there is 

 as yet little certainty. 



Vascular System. 



The simple tubular heart (Fig. 129, H.} lies in a peri- 

 cardial space at the side of the lower end of the pharynx. 

 In development, two diverticula grow out from the pharynx; 

 these meet and fuse, forming the pericardium. The heart 

 arises as an evagination from its dorsal wall. According to 

 some authorities, the cavities of the heart and of the blood 

 vessels are blastocoelic in origin, i.e., they are said to be 

 derived from the segmentation cavity of the embryo. A 

 periodical reversal of the direction of the waves of contrac- 

 tion is discernible in the heart ; for a certain number of beats 

 the blood is driven upwards, and then the direction is 

 reversed. This is said to be, at any rate partially, due to 

 the differences in oxygenation of the blood at the two ends 

 of the heart. This same reversal also occurs in Phoronis. 



According to Professor Herdman, the ventro-dorsal contractions 

 occasion the following circulation. The blood, which is spread out on 

 the walls of the pharynx in vessels lying between the slits, collects into 

 one large vessel, which, after receiving a vessel from the tunic, enters 

 the ventral end of the heart. From the dorsal end it is poured into a 

 great trunk, which sends one branch to the tunic and then breaks up 

 among the viscera. From the visceral lacunae the blood is again collected 

 to be distributed to the branchial sac. At the reversal of the contrac- 

 tions this circulation is also reversed. The blood is very colourless, but 

 usually contains a few pigmented corpuscles. 



Excretory System. 



In the loop of the intestine there lies a mass of clear 

 vesicles containing uric acid and other waste products. 

 This, therefore, seems to be a renal organ, but there is no 

 duct. Bacteria are usually found in the vesicles, and their 



