Metazoa A mphioxns. 255 



the body above the level of the branchial slits. These folds 

 of skin grow downwards until they have met on the ventral 

 surface of the body. They then fuse all along the middle 

 line, save opposite the posterior end of the pharynx, where 

 they leave a small aperture already mentioned the atrial 

 pore. In front these folds form the side -walls of the buccal 

 cavity. The cavity thus enclosed the atrial cavity must 

 not be confused with the body-cavity, or ccelom, afterwards 

 to be mentioned. 



Alimentation and respiration, The primary stages 

 in these two processes take place simultaneously, and by 

 means of the same organ. The sea- water containing minute 

 organisms enters the buccal cavity, being induced to do so 

 by the incessant action of the cilia on the cells lining the 

 cavity. The food particles are then collected from the 

 water and enter the intestine. The water meantime escapes 

 through the branchial slits, and in streaming over the bran- 

 chial arches carries fresh oxygen to, and removes carbonic 

 acid from, the blood circulating in spaces (lacunae) in the 

 branchial arches. The water escapes from the atrial chamber 

 by the atrial pore. The food matter, after being mixed with 

 the liver secretion and undergoing digestion in the wider 

 portion of the alimentary canal, is absorbed into the circu- 

 lation in the intestine proper. 



The circulatory system. Just beneath the floor of the 

 hypobranchial groove of the pharynx there lies a large blood- 

 vessel, the cardiac aorta, which gives off a series of lateral 

 branches, which enter into the branchial arches, and there 

 break up into smaller vessels. On each lateral branch, just 

 before it enters the arch, there is a small dilatation, which 

 has the power of contracting rhythmically, known as a bran- 

 chial heart. The blood, which is colourless and contains 

 very few corpuscles, is collected again from the branchial 

 lacunae into two vessels, one on either side, situated close to 

 the sides of the ridges of the pharynx which form the walls 

 of the hyperbranchial groove. These are the two dorsal 



