178 COMPARATIVE ANATOMY 



In preserved specimens, in which the muscles are contracted 

 by the action of spirit, the gill-slits are not vertical, but their 

 lower ends are directed backwards at a sharp angle so that 

 many of them are cut across in transverse section. The slits 

 are separated from one another by narrow but rather deep 

 partitions called gill-bars, and as the gill-slits are formed as 

 outgrowths of the gut which have met, fused with, and 

 eventually broken through the epiblast (here the epiblast 

 lining the atrial chamber), each gill-bar is covered externally 

 with epiblastic epithelium, internally and at the sides, by 

 hypoblastic epithelium, and contains a core of mesoblast. 

 The gill-bars are strengthened by a series of chitinoid 

 skeletal rods whose arrangement is shown in fig. 42, C. 

 The dorsal end of every rod curves over to join the rod 

 next behind it, but the ventral extremities are not connected 

 together, and end alternately in simple and bifurcated points. 

 The gill-bars which have bifurcated skeletal rods are known 

 as primary bars, those which have simple rods are known as 

 secondary or tongue-bars, and the two gill-slits lying between 

 two primary bars together constitute a primary gill-cleft. At 

 the time of its first appearance every primary cleft was a 

 simple oval opening. After a time a tongue-like downgrowth 

 was formed from its dorsal margin, which grew across the 

 cavity of the cleft until it met and fused with its ventral 

 margin, thus dividing it into two. The primary bars, then, 

 are the original partitions between the primary clefts, the 

 tongue-bars are secondary growths which divided the primary 

 clefts into two. There are other differences between the 

 primary and secondary bars which will be described later, 

 after the ccelom has been treated of. The primary bars are 

 connected together by several cross-bars or synapticula, 

 which at first skip over, but eventually are fused to the 

 tongue-bars. The dorsal wall of the pharynx is indented 

 by a deep but narrow groove, the hyperbranchial groove, 

 running along its whole length close beneath the notochord. 

 The sides of the groove are covered with a special ciliated 

 epithelium, which, at the anterior end of the pharynx, is 

 continued right and left into two ciliated tracts, the peri- 

 pharyngeal bands. These bands curve downward and join 

 the anterior end of a ventral groove which runs along the 

 whole length of the floor of the pharynx. This ventral groove 



