468 Simon Hen^y Gage 



as sometimes happens, the respiration of the animal could 

 still be carried on. In other words having 14 practically in- 

 dependent gill pouches renders the liability to suffocation far 

 less than if a single entrance or exit served for the entire 

 respiratory supply. 



Since in the adult, the inspiratory stream must enter the 

 same opening from which the expiratory stream emerges, 

 there must be a different arrangement of valves from that ob- 

 taining in the larva, where the branchiopores serve only for 

 the exit of the water. The single valve of the larva is 

 present in the adult, but it is not wide enough to cover the 

 entire branchiopore as in the larva ; usually it covers only 

 about the cephalic half (PI. VIII, fig. 55). 



Inspiration is effected largely in both adult and larva by 

 the elasticity of the cartilaginous branchial basket work, and 

 expiration through the constriction of the branchial ap- 

 paratus by muscular action, thus standing in marked con- 

 trast to the respiratory actions of mammals where the thoracic 

 cage must be expanded by active muscular contraction for in- 

 spiration, while expiration is largely effected by the elasticity of 

 the respiratory apparatus. 



In the case of the lamprey one might think at first that no 

 valves are necessary in respiration, for if the branchial pouches 

 are open to the surrounding medium through the branchio- 

 pores any enlargement of the branchial space would cause the 

 water to enter, and conversely, any constriction would empty 

 the branchial sacs. This view is correct, but this mode of 

 simpl}^ drawing water into a sac and expelling it has not ap- 

 parently answered the requirements of the lamprey, and there 

 is present the thin valve (theectal valve) which covers the en- 

 tire branchiopore in the larva (fig. 52-55), and in addition a 

 double valve (ental valve) (fig. 55), which is formed by the 

 growth and modification of the middle gill lamella of the 

 caudal half of the branchial sac. This lamella, near the 

 branchiopore, bifurcates and soon loses its secondary laminae 

 and each part extends laterad as a firm but flexible membrane 

 attached to the caudal wall of the branchial sac, one to the 

 dorsal the other to the ventral edge of the branchiopore and 



