THE PISCES. 63 



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 Inverttbrata. 



XXIV. THE PISCES. 



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 temporary 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 connection with, them are 

 developed villi, or lamellae, which subserve the respiratory 

 function. 



Median fins, formed by prolongations of the integument, 

 supported by one or other kind of skeleton, are very character- 

 istic of fishes ; and it is questionable if any fish exists altogether 

 devoid of the system of median fin-rays and their supports, 

 which have been termed inter-spinous bones and cartilages. 

 On the other hand, no vertebrate animal, other than a fish, is 

 known to possess them. 



When the limbs, or pectoral and ventral fins, of fishes are 



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

 clefts is not known. 



