THE LOWER VERTEBRATA. 207 



are ripe, they simply burst through the wall of the pouch 

 into the atrium, whence they escape by the atriopore. 

 What then becomes of them we shall see shortly. 



15. The Principle of Change of Function. The student 

 is now in a good position to realize how in different types 

 the same structures are turned to different purposes. The 

 gill-slits and arches in Amphioxus serve primarily to strain 

 off the water that brings in the food, and as a secondary 

 function they support respiration. In the dogfish the first 

 function has gone, and the main one is that of respiration, 

 but the basal parts of the cartilages serve for the attach- 

 ment of respiratory muscles, while it seems probable that 

 the jaw-apparatus is really the first of the cartilaginous 

 arches turned to a new use. In the frog, the respiratory 

 use of the slits is lost, and they themselves are gone, except 

 the first, which is turned to a new purpose in connexion 

 with the ear, forming the tympanum ; the basal cartilage 

 remains useful for the respiratory muscles, but it also serves 

 a new function the support of the tongue-muscles. Finally, 

 in the rabbit the remains of the branchial arches (" hyoid 

 apparatus") serve for the tongue-muscles and the larynx 

 only, and have nothing to do with respiration. 



With this example of gradual change of function in view 

 (and it has not been put exhaustively), the student will 

 be less surprised than he would otherwise be when he is 

 told that the thyroid of higher Vertebrates represents the 

 endostyle of Amphioxus. The thyroid, in fact, starts its 

 development as a groove in the floor of the pharynx, but 

 this groove (not needed in an animal with jaws) becomes 

 converted into a canal and separated off from the pharynx. 

 Its varying position in the higher types is dependent upon 

 adaptation to the varying growth of other organs. Whether 

 the endostyle of Amphioxus secretes the same internal 

 secretion as the thyroid of higher types is not known ; 

 but it is reasonable to suppose that if we had a series of 

 organisms forming as complete a chain between Amphioxus 

 and the fishes as we have between fishes and mammals, 

 we should find a gradual decrease in the importance of 

 the primary function of the endostyle accompanying a 



