416 CEPHALOCHORDA, AMPHIOXUS. 



Respiratory System. 



The water which enters the mouth and passes down 

 the pharynx leaves this by the numerous gill slits. In 

 the embryo these open directly to the exterior; in the 

 adult, into the atrial chamber, which opens by the single 

 atriopore. 



In development two folds appear laterally on the body wall, and form 

 the hollow metapleural folds of the adult. On their inner apposed, but 

 not united, surfaces, two ridges appear. These grow towards one 

 another and unite, leaving only the atriopore open. Thus the floor of 

 the atrial chamber (Fig. 133, atr. ) is produced. The chamber, as first 

 formed, is a tube with a very small lumen. Secondarily, it becomes 

 enlarged, constricting the body cavity, as we noticed above, until it 

 comes almost to surround the gut. At the same time, the metapleural 

 folds increase in size until they assume the adult appearance 

 (Fig. 133, mp.}. The water currents are kept up by the action of cilia, 

 and by the movements of the transverse muscles. 



The gill slits gradually become more numerous as the animal grows 

 older, and in the adult there are more than a hundred. 



The original number of gill slits is doubled by the growth of a 

 secondary bar down the centre of each slit, thus producing two gill slits 

 where there was formerly only one. The primary bars are distinguished 

 from the secondary in being split, and there are also histological 

 differences between them. 



Circulatory System. 



The blood is colourless, with a few amoeboid cells. 

 There is no definite heart, but the vessels are said to be 

 contractile in several places. 



Vessels from the body and from the caecum unite to form a ventral 

 vein, the cardiac aorta, which runs forward beneath the pharynx. From 

 this vessel a series of smaller vessels arise, which pass up the primary 

 branchial rods, and are termed aortic arches. The most anterior of the 

 right side is larger than the rest, and sends branches to the head region. 

 The aortic arches apparently open into two dorsal vessels, the right and 

 left dorsal aortoe, which unite at the hinder end of the pharynx to form 

 a single vessel running backward above the intestine. The blood 

 vessels which are supposed to take blood from the intestine to the liver 

 are termed portal veins ; those passing from the liver to the ventral vein 

 are termed hepatic. The portal vein and the cardiac aorta are said to 

 be specially contractile. 



Although the names given above to the various vessels indicate the 

 views generally held as to the course of the blood, our knowledge of this 

 is only hypothetical. 



Excretory System. (a) Quite recently Prof. Boveri has discovered 

 nephridial tubes in the adult. "These are found in the region of the 



