INTERNAL ANATOMY. 



33 



bar (Fig. 15 B}, commencing from the outside, that is to 

 say, from the edge turned towards the atrial cavity, we 

 have first a patch of columnar atria) epithelium, at the cor- 

 ners of which some of the cells contain a quantity of the 

 rich brown pigment which has been referred to above as 

 being characteristic of the atrial epithelium generally. 



Fig. 15, A and B. Transverse sections through primary (B) and secondary 

 (.4) gill-bars. (After BENHAM, slightly altered.) 



a.e. Atrial epithelium, b.e. Branchial epithelium, c. Ccelomic space of primary 

 bar. sk. Skeletal rods. v. Ccelomic vessel of primary bar. v". External vessel 

 of both bars, v'" . Internal vessel of both bars. 



N.B. Benham holds the space at the inner edge of the skeletal rod of 

 tongue-bar for a blood-vessel. 



C. Isolated ciliated cells of the branchial epithelium. (After LANGERHANS.) 



Next comes a cavity which is a portion of the coelom, and 

 is lined by the flat ccelomic epithelium. In fact, the dor- 

 sal, or subchordal ccelom on each side (cf. Fig. 2) is put 

 in connection with the endostylar coelom by a canalicular 

 detachment of the ccelom which accompanies each primary 



