i82 COMPARATIVE ANATOMY 



perforating the side walls of the pharynx, and placing its 

 cavity in communication with the exterior. In the natural 

 uncontracted condition of the young animal these slits 

 would be vertical,, and would be separated from one another 

 by vertical partitions, narrow from front to back, but deep 

 from side to side. As -the gill-slits are confined to the side 

 walls of the pharynx, the coelomic space below the floor of 

 the pharynx would be unaffected by their presence, as would 

 also be the case with the dorsal spaces right and left of the 

 middle line. But in the side walls the coelom would be re- 

 duced to a series of narrow passages running down the 

 partitions separating the primary gill-slits — i.e. the primary gill- 

 bars. And, in fact, we find in the adult that there is a 

 triangular space right and left of the upper part of the 

 pharynx, usually known as the dorsal ccBlom, which com- 

 municates by a narrow canal running down each primary gill- 

 bar with a space underlying the endostyle, and hence known 

 as the subendostylar coelom (fig. 44, S.cd). 



It has already been explained that each primary gill-slit is 

 divided into two by a tongue-like downgrowth from its upper 

 margin. The coelom does not extend into these secondary 

 downgrowths, and consequently the tongue-bars do not con- 

 tain coelomic canals, while the primary bars do, a difference 

 which is obvious enough in a cross section through the gill- 

 bars. On the formation of the atrial cavity the shape of the 

 dorsal coelomic spaces is somewhat altered, and curious 

 changes occur in connection with the development of the 

 gonads, but the main relations described above are undis- 

 turbed, except, of course, that the gill-slits no longer open to 

 the outside but into the atrial chamber. Owing to the con- 

 traction of the muscles of adults preserved in spirit the gill- 

 slits slope backwards, -and it is because many gill-bars are cut 

 through that transverse sections through the pharynx have 

 so curious and complicated an appearance. The complexity, 

 however,- may be readily understood by a study of the plastic 

 diagram given in fig. 44, which has been fully lettered to save 

 lengthy description. Fig. 45, C and D, are transverse sections 

 through a primary and a tongue-bar, showing the coelomic 

 canal in the former and its relation to the skeletal rod. It 

 may further be seen that there are three blood-vessels in a 

 primary bar, but only two in a tongue-bar. 



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