21. 



and s'^70llen, all of tlie plates are alike in shape and structixre. 

 Laterally tlie plates of eacii gill lie opposite each other. Gen- 

 erally in the length of each gill, one or more plates are inter- 

 polated on one side or the other, figure 20, ip, and thus the 

 plates are made to alternate for a short distance, but they soon 

 become opposite again. 



The plates on the two sides of each gill are separated 

 from each other except belov^, -viftiere they are fused, either leav- 

 ing a connecting ridge, as frequently happens anteriorly, or a 

 smooth interiiediate portion, as is generally the case further 

 back. 



Each plate is also Joined to the s^ispensory nembrariC, 

 figure 19 su, abo\'-e, and to two longitudinal muscles, 11m and ulm, 

 that are continuous the length of the gill. Each plate is hollow 

 and may be likened to a loosely folded sheet of paper, leaving 

 a space between ; -vfoich space is traversed by a few fibers of 

 connective tissue. Ihe unshaded line, y, figure 19, represents 

 1/fliere the wall has been cut in separating the plate from the 

 plate lying in front of it. liie opposite wall of the plate is 

 continuous, in a like manner, with the wall of the plate lying 

 behind. Along each side of the ventral portion of each plate. 



