428 THE LANCELET CHAP. 



fine roots, and arise from the ventro-lateral angles of the cord. The 

 dorsal and ventral nerves are not in the same transverse plane (compare 

 Fig. 52), but have an alternating arrangement. 



5. The gelatinous connective-tissue forming the fin-rays. 



6. The intestine, with its single layer of long epithelial cells. If the 

 section passes through the anus, the latter will be seen opening on the 

 left side of the ventral fin. 



7. The ccelome, surrounding the intestine, and also enclosing the 

 subintestinal veins, which are continuous anteriorly with the hepatic 

 portal vein (Fig. 1 10). 



8. The dorsal aorta, just below the notochord. 



b Go over 2, 3, 4, 5, 6, and 8 again all much as in a. Note 



1. The dorsal and lateral (metapleural} fin-folds, ventral body- 

 wall, small metapleural canals, and atriopore if present in your 

 section. 



2. The ccelome, a narrow space round the intestine, in which three or 

 four subintestinal (portal] veins can be seen. 



3. The atrial cavity just outside the ccelome, and separated from it 

 by the atrial membrane. If the section passes through the atriopore, 

 it will be seen in the mid-ventral line, putting the atrial cavity in com- 

 munication with the exterior. 



c. Note I. The triangular form of the body in transverse section ; 

 the dorsal fin, the ventral surface, with its folded integument and thin 

 transverse ventral muscles ; the metapleural folds and their contained 

 large canals. Then go over a. 2, 3, 4, and 5 again, which are all much 

 as in a and b. 



2. The pharynx, lying just below the notochord, and extending nearly 

 to the ventral body walls. Note (a) the deep, dorsal epibranchial 

 groove ; (b) the ventral endostyle projecting into the pharyngeal cavity, 

 and enclosing a ccelomic canal ; (c] the gill-bars, a great number of 

 which will be cut through obliquely, larger primary alternating with 

 smaller secondary ones. Note the skeletal rods near their outer surfaces, 

 and, in the primary bars, the small ccelomic spaces on the outer sides of 

 the rods ; (d) the dorsal siispensory folds of the pharynx, which enclose 

 the two dorsal ccelomic canals. 



3. The atrial cavity, surrounding the pharynx, except at the mid- 

 dorsal line where the pharynx is attached to the sheath of the noto- 

 chord, and communicating with the cavity of the pharynx through the 

 gill-slits. It is enclosed by the two atrial folds, which are united in 



