228 LECTUKE IX. 



haps, the sole examples of 'permanent teeth' to be met with in the 

 whole class. 



When the teeth are developed in alveolar cavities, they are suc- 

 ceeded by others in the vertical direction (V. pi. AQ.Jig. 1.) : these owe 

 the origin of their matrix to the budding out from tlie capsule of 

 their predecessors of a cascal process, in which the papillary rudiment 

 of the dentinal pulp is developed according to the laws explained in 

 V (Introduction). But, in the great majority of Fishes, the germs 

 of the new teeth are developed, like those of the old, from the free 

 surface of the buccal membrane throughout the entire period of suc- 

 cession ; a circumstance peculiar to the present class. The Angler, 

 the Pike, and most of our common Fishes, illustrate this mode of 

 dental reproduction : it is very conspicuous in the Cartilaginous 

 Fishes (V. pi. 5, Jig. 1.), in which the whole phalanx of their 

 numerous teeth is ever moving slowly forwards in rotatory pro- 

 gress over the alveolar border of the jaw, the teeth being successively 

 cast off as they reach the outer margin, and new teeth rising from the 

 mucous membrane behind the rear rank of the phalanx. 



This endless succession and decadence of the teeth, together with 

 the vast numbers in which they often coexist in the same Fish, illus- 

 trate the law of Vegetative or Irrelative Repetition, as it manifests 

 itself on the first introduction of new organs in the Animal King- 

 dom, under which light we must view the above-described oi^ganised 

 and calcified preparatory instruments of digestion in the lowest class 

 of the Vertebrate series. 



Alimentary Canal. 



The mouth of Fishes is the common entry and vestibule to both 

 the digestive {Jig. 61. dton) and the respiratory (ib. t, u) organs ; it 

 is, therefore, of great capacity : and, as the transmission of the food 

 to the stomach, and of the respiratory currents to the gills, is per- 

 formed by similar acts of deglutition, the bony arches which surround 

 the mouth are not only large, but are complicated by a mechanism 

 for regulating the transit of the nutritious and oxygenating media, 

 each to their respective localities. The branchial slits are provided 

 with denticles and sieve-like plates or processes to prevent the 

 entry of food into the interspaces of the gills, and the branchial out- 

 lets are guarded by valves which reciprocally prevent the regurgita- 

 tion of the respiratory streams back into the mouth. 



The necessary co-operation of the jaws with the hyoid arch in the 

 rythmical movements of respiration is incompatible with protracted 

 maxillary mastication ; and, accordingly, the branchial apparatus 

 renders a compensatory return by giving up, as it were, the last pair 

 of its arches to the completion of the work which the proper or 



