428 THE LANCELET CHAP. 



4. The spinal cord, lying in the neural canal. It is ovoid 

 in transverse section, and has a dorsal fissure extending 

 downwards nearly to the central canal, which is nearer the 

 ventral than the dorsal surface. See if your section happens 

 to pass through a dorsal or ventral nerve. The dorsal nerves 

 arise by large single roots from about the middle of the sides 

 of the cord, while the ventral nerves have numerous 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 arrange- 

 ment. 



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 coelome, surrounding the intestine, and also 

 enclosing the subintestinal veins, which are continuous 

 anteriorly with the hepatic portal vein (Fig. no). 



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, and small metapleural canals. 



2. The coelome , a narrow space round the intestine, in 

 which three or four subintestinal (portal] veins can be seen. 



3. The atrial cavity just outside the coelome, 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 communication 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 1 

 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 coelomic 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 



