SECTION THROUGH ATRIAL PORE 193 



c. The hepatic veins are three or four vessels lying 

 side hy side on the dorsal surface of the liver, in 

 the coelomic space hetween the liver and the atrial 

 membrane investing it. 



7. 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 passing through the Atrial Fore. 



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. 

 The metapleural folds are present, but are much 

 smaller than in B or 0, and between them in the 

 median plane is the atrial pore. 



The skin, and the skeletal, muscular, and nervous 

 systems are practically the same as in B, except that 

 the ventral muscles are absent. 



2. The intestine is circular in section. Its waUs are thick 



and, like those of the liver, consist of a single layer 

 of very long and slender epithelial cells, resting on 

 a thin connective-tissue basement membrane. 



3. The atrial cavity surrounds the sides and the ventral 



surface of the intestine, and opens to the exterior at 

 the atrial pore in the mid- ventral hne between a pair 

 of thickened and prominent hps, which are prolonged 

 backwards like a spout. 



4. The coelom is reduced to a narrow space surrounding 



the intestine, between its proper wall and the atrial 

 membrane. 



5. The metapleural canals are very greatly reduced in size. 



6. The circulatory system. 



a. The dorsal aorta is a median vessel, between the 

 notochord and the intestine. 







