174 



Subkingdom VII. VERTEBRATA. 



MYELENCEPHALA. 



Red-blooded animals, with the mass of the nervous centres 

 enclosed in a bony axis [cerebro-spinal system]. Sexes always 

 distinct. 



The cerebro-spinal system is not represented among inver- 

 tebrated animals, their ganglionic system being only the homo- 

 logues of the vertebrate sympathetic system. Among invertebrates 

 there is only one general cavity, in which the viscera and circula- 

 tory organs are contained. This is the " haemal " region. In ver- 

 tebrates a "neural" region, containing the great nervous masses, 

 is also present. In the embryo the two cavities are developed 

 at an early period. 



Another character in which the vertebrate embryo differs from 

 all others is in the possession of a "notochord " or "chorda dor- 

 salis ;" this is found in early embryonic life before the cerebro- 

 spinal axis is complete ; it is a rod-like body, the " substance of 

 the centre of the floor " of the spinal column, by which in most 

 cases it is replaced. An amniou and allantois, foetal membranes, 

 are confined to reptiles, birds, and mammals. 



Vertebrates only have true teeth ; these are quite distinct from 

 bone, and belong to the dermal appendages. The jaws are in- 

 variably above and below ; never on each side. The muscles are 

 always external to the bones. 



Owen divides the Vertebrata into Hgematocrya and Haemato- 

 therma cold- and warm-blooded animals respectively. Huxley 

 recognizes three primary divisions Mammalia, Sauropsida (com- 

 prising birds and reptiles), and Ichthyopsida (amphibia and fishes). 

 Hackel has four " main classes " Leptocardia or Acrania; Mono- 

 rhina, lampreys only ; Anauinionata (=Anallontoidea), fishes and 

 amphibia; and Amnionata (=Allantoidea), reptiles, birds, and 

 mammals, the last comprised in Pachycardia or Craniota ( = Holo- 

 crania). Ray Lankester, adopting Leptocardia (but with the name 

 of Cephalochorda) and Craniota, includes also Urochorda(=Tu- 

 nicata) in the Vertebrata. 



The five universally recognized classes are : 



Cold-blooded ; heart with less than two auricles and 



two ventricles. 

 Gills present, at least in the earlier stages. 



No lungs PISCES. 



Lungs in the adult AMPHIBIA. 



