62 ON CLASSIFICATION. 



tion, these are either hard productions of the alimentary mucous 

 membrane, or are modified limbs. In the latter case there 

 may be many pairs of them — numerous Crustacea, for example, 

 have eight pairs of limbs devoted to this function. In no verte- 

 brated animal, on the other hand, are limbs so modified and 

 functionally applied, the jaws being always parts of the cephalic 

 parietes specially metamorphosed, and totally distinct in their 

 nature from the limbs. All vertebrated animals, finally, possess 

 a distinct vascular system, containing blood with suspended 

 corpuscles of one kind, or of two, or even three, distinct kinds. 

 In all, save one, there is a single valvular heart — the vessels of 

 the exception, Amphioxus, possessing numerous contractile dila- 

 tations. All vertebrates possess a " hepatic portal system," the 

 blood of the alimentary canal never being wholly returned 

 directly to the heart by the ordinary veins, but being more or 

 less largely collected into a trunk, the " portal vein," which 

 ramifies through and supplies the liver. 



These are the most important characters by which the verte- 

 brate classes are distinguished, as a whole, from the other classes 

 of the animal kingdom ; and their number and importance go a 

 long way to justify the step taken by Lamarck when he divided 

 the animal kingdom into the two primary subdivisions of Verte- 

 brata and Invertebrafa. 



If we seek now to construct definitions of the first two classes 

 of the Vertebrata, Pisces and Amphibia, we shall meet with 

 some difficulties, arising partly from the wide variations ob- 

 servable in the structure of fishes, and partly from the close 

 affinity which exists between them and the Amphibia. 



No fish exhibits any trace of that temj^orary appendage of 

 the embryo of the higher vertebrates which is termed an amnion, 

 nor can any fish be said to possess an allantois, though the 

 urinary bladder of fishes may possibly be a rudiment of that 

 structure. The posterior visceral clefts and arches * of fishes 

 persist throughout life, and are usually more numerous than in 

 other vertebrates ; while upon, or in connexion with, them are 



* The relation of the perforated pharynx of Ampkioxus to the visceral arches and 

 clefts is not known. 



