50 PHYLUM VERTEBRATA (CRANIATA). 



never present any trace of segmentation.* In the female they 

 retain this relation throughout life (except in Teleostei), but 

 in the male the generative part of the coelomic epithelium 

 always (except in Marsipobranchii) loses its relation with the 

 general body-cavity in the adult. 



The ovum varies considerably in character in the different 

 classes. In Pisces (except Elasmobranchii) and in Amphibia 

 it is comparatively small and holoblastic,f and the young are 

 always hatched out in an immature condition as larvae. In 

 Elasmobranchii, Reptilia, and Aves the ovum is large and 

 meroblastic, and the young when hatched resemble the adult, 

 a larval stage being absent. In Mammalia the egg is smaller 

 than in any other Vertebrate, and except in one class under- 

 goes almost the whole of its development in the oviduct, the 

 young being born in a condition closely resembling the adult. 

 An amnion, allantois and primitive streak are found in the 

 embryos of all Reptiles, Birds and Mammals, but are absent 

 from all Pisces and Amphibia. 



The division of the Vertebrata into the four classes Pisces, 

 Amphibia, Aves, and Mammalia was first established by 

 Linnaeus, though it had already been indicated in the system 

 of Aristotle. The Pisces and Amphibia are cold-blooded 

 animals (i.e. animals with a varying temperature) ; Aves and 

 Mammalia are warm-blooded. Since Linnaeus' day, his group 

 Amphibia has been split up into the naked Amphibia and into 

 the scaly animals or Reptilia. Pisces and naked Amphibia 

 have many characters in common, e.g. the branchial respiration, 

 the frequent persistence of the notochord, the absence of an 

 amnion and allantois, etc. On these grounds and in con- 

 sideration of the many relations between Reptiles and Birds, 

 Huxley has distinguished three principal groups of Vertebrata 

 the Ichthyopsida (Pisces and Amphibia), the Sauropsida 

 (Reptilia and Aves), and the Mammalia. 



* See note on p. 88. 



f In Teleostei the ovum though small is meroblastic. 



