142 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



birds, the duckbill and the echidna, and some of 

 the whales among Mammals; this is a phenomenon 

 not confined to Vertebrata, for it may be observed in 

 the Spatangoids among Echinoidea, where the 

 " Lantern of Aristotle " is altogether absent, and in 

 the tubicolous Chsetopods, which have lost the 

 strong jaws of their free-swimming allies. 



In correspondence with the great diversity of 

 mode of life and of details of structure among fishes, 

 we note in that group the very greatest differences 

 in the disposition and size of the teeth; seeing, indeed, 

 here an excellent illustration of the law that com- 

 mencing structures are subject to great variability. 

 Here, too, we find an example of spines on the skin 

 taking on the function of teeth ; the true teeth, that 

 is to say, the hard structures within the area of the 

 mouth, are, in the saw-fish (Pristis), quite small and 

 blunt ; the sides of the enormous snout are, however, 

 provided with large dermal spines, set at regular 

 distances from one another, and each implanted in a 

 special socket. 



When well developed, as in the dog-fish, the teeth 

 are set in several concentric rows ; those of the outer 

 are alone functional, and they, as all, are not 

 attached to the jaw, but are only fixed in 

 the covering membrane ; this membrane appears 

 to move over the surface of the jaw, and 

 thereby the teeth which have been in use for a time 

 are removed from the edge of the jaw, and the 

 next succeeding series come to occupy their position, 

 and to take on their function. 



A large number of small teeth are likewise to be 

 found in many bony fishes (Teleostei), and here, where 

 a number of distinct bones are developed^ we often 

 find every bone within the mouth bearing teeth ; as 

 may readily be supposed, such teeth are generally 

 of small sizej and without any special masticatory 



