192 AMPHIOXUS 



in contact with each other ; and the hypo- 

 branchial groove is as well marked as the hyper- 

 branchial, 

 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. 



3. The atrial cavity has the same general relations as in 



B. The atrial epithelium covers the outer surfaces 

 of the branchial bars, and the suspensory folds of 

 the pharynx ; and is continued downwards over 

 the inner surfaces of the atrial folds, covering the 

 reproductive organs, and being thrown into folds 

 along the ventral wall of the atrial cavity. It also 

 forms an investment to the liver. 



4. The coelom is still more subdivided than in B : its main 



divisions are as follows. 



a. The dorsal coelomic canals are a pair of large spaces 



above the suspensory folds of the pharynx, as in B. 



b. The branchial coelomic canals are a series of cavi- 



ties in the primary branchial bars, as in B. 



c. A space surrounding the liver, between its proper 



wall and the atrial epithelium, is part of the 

 coelom. 



d. Paired spaces immediately surrounding the re- 



productive organs are also extensions of the 

 coelom. 



5. A pair of large metapleural canals lie in the lateral fins. 



6. The circulatory system. 



a. The cardiac aorta, the dorsal aortae, and the aortic 



arches are the same as in B. ' ^ 



b. The portal veins He on the liver, in the coelomic 



space surrounding it. They are often difiSc'ult to 

 identify in transverse sections. 



