188 AMPHIOXUS, 



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. 41 and 42, 

 into the large dorsal ccelomic canals above the 

 suspensory folds. Branchial coelomic canals also 

 traverse the skeletal rods of the secondary gUl- 

 arehes. 



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



the cardiac aorta runs, below the endostyle. The 

 branchial coelomic 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 be 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, running 



forwards along the ventral wall of the pharynx in the 

 endostylar coelomic 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 coelomic, the somatic, and the 

 visceral. The coelomic 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 coelomic 

 canal, while in the secondary bars it appears to lie 

 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 edge of the 



