196 AMPHIOXUS. 



the folds into which the atrial epithelium is 

 thrown along the outer sides of these arches. 

 These cavities are very small in the ventral 

 portions of the arches, but widen out towards 

 their dorsal ends, and open, as shown in figs. 43 

 and 44, into the large dorsal ccelomic canals 

 above the suspensory folds. Branchial ccelomic 

 canals also traverse the skeletal rods of the 

 secondary gill-arches. 



c. The endostylar ccelomic canal is a space, in which 



the cardiac aorta runs, below the endostyle. 

 The branchial ccelomic canals of both primary 

 and secondary gill-arches communicate with it. 



d. A series of perigonadial ccelomic spaces surround 



the reproductive organs. 



G. The Circulatory System. 



The main features of the circulatory system can he made 

 out by examination of a series of transverse sections. 



There is no heart, but the general course of the circulation 

 is the same as in other gill-breathing vertebrates. The blood 

 is colourless and contains but few corpuscles. 



The principal vessels are as follows : 



1. The cardiac aorta is a median longitudinal vessel run- 



ning forwards along the ventral wall of the pharynx 

 in the endostylar ccelomic canal. 



2. The aortic arches are a series of vessels running up the 



pharyngeal bars or gill-arches. There appear to be 

 three in each bar, the ccelomic, the somatic, and the 

 visceral. The ccelomic aortic arch is the outermost 

 of the three, and in the primary bars lies close to 

 the atrial epithelium in the wall of the branchial 

 ccelomic canal, while in the secondary bars it ap- 

 pears to He within the skeletal rod. The somatic 

 aortic arch runs along the inner edge of the skeletal 

 rod. The visceral aortic arch runs near the inner 



