Fig. 54. — Horizontal section belonging to the same series as the last, 

 but at a distinctly deeper level. Some of the gill-arch structures are seen 

 in either head in front, and behind these certain chambers of the double 

 heart, which lies inside a single large pericardial cavity. The alimentary 

 canals are double down to the level of the stomach. Two bile ducts enter 

 the first part of the composite intestine. For references see under PI. II. 

 fig. 7. ( x 20. ) 



Fig. 55. — Horizontal section further back in the same series as the last. 

 The section passes through the sharply curved portion of the body behind 

 the place of union. The spinal cord is cut in two places ; its proximal 

 section is drawn out transversely and is obviously composite, while the 

 distal section is almost normal. Both regions have a pair of notochords. 

 The mesial muscular mass between the notochords is well illustrated here, 

 and is seen to be divided into a series of segments corresponding with the 

 two outer series of muscular segments. The disposition of the adjacent 

 neural arch cartilages is also indicated. References under PI. II. fig. 7. 



(X28.) 



PL. XVI. 



