204 AMPHIOXUS. 



the ventral region of the endostyle. It encloses 

 the cardiac aorta. 



8. The metapleural canals are a pair of large spaces, 



triangular in section, lying in the metapleural folds. 



9. The circulatory system. The larger vessels can be re- 



cognised in sections, by the coagulated blood which 

 they contain. 



a. The cardiac aorta is a median vessel lying in the 



endostylar eoelomic canal. 



b. The dorsal aortse are a pair of vessels lying one on 



each side of the epibranchial groove. 



c. The aortic arches lie in the branchial bars, both 



primary and secondary (see p. 188). 



10. The nervous system is the same as in section A. 



C. Transverse Section passing through the Binder Fart of 

 the Pharynx. 



1. General characters. The section is larger than B, pass- 



ing through the thickest part of the body. It is 

 triangular in shape, the angles being formed by 

 the dorsal and lateral fins respectively. The section 

 passes through the laterally compressed pharynx, on 

 the right side of which is the liver. The reproduc- 

 tive organs project into the atrial cavity from the 

 inner sides of the atrial folds. 



The cutaneous, skeletal, muscular, and nervous 

 systems are practically the same as in section B. 



2. The digestive system. 



a. The pharynx has the same structure as ia section B, 



but differs markedly in shape, being greatly com- 

 pressed laterally, so that its two sides are almost 

 in contact with each other ; both endostyle and 

 epibranchial groove are well marked. 



b. The liver lies at the right side of the pharynx, and 



is oval in section. Its walls are thick, and consist 



