THE DIGESTIVE AND RESPIRATORY SYSTEM 267 



prey and allied forms, which, on account of it, are termed the 

 Cyclostomata. This type is similar to that of Amphioxus 

 and may be related to it; it is circular in shape and equipped 

 with horny, epidermic teeth. 



The second, or gnathostoma, is furnished with a movable ' 

 pair of skeletal jaws, equipped with teeth formed of dentine 

 overlaid with enamel. In origin these jaws are a pair of vis- 

 ceral arches and the teeth are locally modified placoid scales 

 (See Chap. III.), and it may be held either that the gnatho- 

 stoma or jaw-mouth is the same as the first, to which the 

 gill-arches with their associated teeth have become added; or 

 that it is a new opening, originally a pair of gill-slits, which 

 have become fused in the mid-ventral line, and that the first 

 mouth has become lost. In favor of this latter view is the 

 position of the gnathostome in the selachians, where it may 

 be expected to show the most primitive condition, and where 

 it is not at the anterior end but on the ventral side with a 

 long rostrum anterior to it. Later on, as is shown in some 

 ganoids, it attains secondarily an anterior terminal position. 



If from the evidence presented an hypothetical sketch be per- 

 missible it may be allowed that in early vertebrates there was 

 a circular jawless mouth, provided with a hood and situated 

 at the anterior end of the body; that in some form midway 

 between the lamprey eel and the shark the habit arose of 

 seizing and taking in food by the anterior gill-slits, the edges 

 of which, provided with sharp, pointed scales, served better 

 for the retention of their living prey than did the oral hood 

 and horny teeth of the actual mouth. The continuance of this 

 habit would perfect the tools employed, which in this case were 

 movable gill-arches armed with placoid scales, and the new 

 mouth, formed by the ventral fusion of two lateral slits and 

 furnished with superior organs of prehension, entirely usurped 

 the function of the old one, which thus became reduced and 

 finally disappeared. In one point alone, that of position, was 

 the old mouth superior, and the final step in the perfection of 

 the new one was its gradual migration to the anterior end, 

 the various steps in the attainment of which may be seen among 

 the ganoids. 



