PISCES. 129 



(i.e., with a constant temperature), and as they attain a much 

 higher grade of life they are distinguished as the higher Vertebrates. 

 Recently the naked Amphibia have rightly been separated from the 

 scaly animals or Reptilia, and together with the Fishes have been 

 distinguished as lower Vertebrates, in distinction to the Reptiles, 

 Birds, and Mammals, which have been classed as higher Vertebrates. 

 Fishes and Amphibia have, in fact, many characters in common, and 

 seem to be less sharply marked off from one another (Dipnoi) than 

 are the Amphibia from the Reptilia. The two former groups not 

 only resemble one another in the branchial respiration and in the 

 frequent persistence of the notocord, but also in the simpler course 

 of the embryonic development and in the absence of the embryonic 

 organs characteristic of the higher Vertebrates the amnion and the 

 allantois. On these grounds, and in consideration of the many 

 relations between Reptiles and Birds, Huxley distinguishes three 

 principal groups of Vertebrata the Ichthyopsida (Pisces and Am- 

 phibia), the Sauropsida (Reptilia and Aves), and the Mammalia. 

 Among the Fishes there are certainly such wide differences of organi- 

 sation that we are justified in dividing them into several classes. 

 The Leptocardia might be separated not only from all the Fishes but 

 also from all other classes of Vertebrates as Acraniata ; also the 

 Selachians, the Cyclostomes and the Dipnoi might be regarded as 

 separate classes if it were not more convenient to preserve the unity 

 of the class Pisces. 



CHAPTER V. 

 Class I PISCES 



Cold-blooded, generally scaly, aquatic animals with unpaired fins 

 and paired pectoral and pelvic fins. They breathe exclusively by 

 means of gills, and have a simple heart consisting of auricle and 

 ventricle. They are without anterior urinary bladder. 



The peculiarities which the structure and internal organisation of 

 these animals present result in general from the requirements of their 



* Cuvier et Valenciennes, "Histoire Naturelle des Poissons." 22 Vole., 

 Paris, 1828-49. 



Job. Miiller, " Vergleichende Anatomic der Myxinoiden. " Berlin, 1835-45. 

 L. Agassiz, " Kecherches sur les poissons fossiles." Neufchatel, 1833-44. 

 Giinther, " Catalogue of the fishes in the British Museum." London. 

 C. E. v. Baer, " Entwickelungsgeschichte der Fische." Leipzig, 1835. 

 vOL. II. 9 



