188 AMPHIOXUS 



8. The circulatory system. The vessels can be readily re- 



cognised in sections, owing to the coagulated blood 

 which they contain. 



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



floor of the pharynx. 



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



each side of the hyperbranchial groove. 



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



primary and secondary, .along the inner sides of 

 the skeletal rods. 



9. The nervous system is the same as in A. 



C. Transverse Section through the Hinder Part 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 B. 



2. The digestive system. 



a The pharynx has the same structure as in B, but 

 differs markedly in shape, being strongly com- 

 pressed laterally, so that its two sides are almost 

 in contact with each other ; and the hypo- 

 branchial groove becomes as well marked as the 

 hyperbranchial . 



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

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

 of a single layer of very long and slender flagellate 

 cells, resting on a thin outer wall of connective 

 tissue. 



