190 AMPHIOXUS 



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 pair of large metapleural canals lie in the lateral 



fins. 



d. A space surrounding the liver, between its proper 



wall and the atrial epithelium, is part of the 

 coelom. 



e. Paired spaces immediately surrounding the re- 



' productive organs are also extensions of the 

 ccelom. 



5. The circulatory system. 



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



arches are the same as in B. 



b. The portal veins lie on the liver, in the coalomic 



space surrounding it. They are often difficult to 

 identify in transverse sections. 



c. The hepatic veins are three or four vessels lying _ 



side by side on the dorsal surface of the liver, in*- 

 the coslomic space between the liver and the atrial 

 membrane investing it. 



6. The reproductive organs occupy similar positions in the 



two sexes. They are masses, varying in size accord- 

 ing to the degree of ripeness of the ova or spermatozoa, 

 lying on the inner sides of the atrial folds, and pro- 

 jecting into the atrial cavity. 



D. Transverse Section through the Atrial Pore. 



1. General characters. The section is smaller than the pre- 

 ceding one, and the myotomes reach much further 

 down the sides. The intestine is cut across, but the 

 section lies entirely behind the reproductive organs. 



