226 VERTEBRATE ANIMALS. 



their most important function, as the retention of this substance 

 within the body rapidly causes death. The secretion of the kidneys 

 is sometimes got rid of by means of special canals appropriated to 

 this alone ; but in the lower Vertebrata it is discharged into the 

 hinder extremity of the alimentarj' canal, and is evacuated along 

 with the undigested portions of the food. 



The nervous system varies greatly in its development in the Verte- 

 brata. In the little fish called the Lancelet, the main mass of the 

 nervous system consists of a cord of nervous matter, representing the 

 spinal marrow, but not having in front any enlargement which repre- 

 sents the brain. In all the other Vertebrata the central masses of 

 the nervous system (termed the eerehro-spinal axis) consist of a nerv- 

 ous cord (the spinal cord) contained in the canal formed by the 

 neural arches of the vertebrse, and of an anterior mass of nervous 

 matter, which is protected by the skull, and is termed the " enceph- 

 alon," or brain. The size and develojiment, however, of the brain 

 vary enormously in different Vertebrates ; and in the lower forms 

 the brain is little more than an aggregation or collection of nervous 

 masses or " ganglia," of which some are connected with the special 

 senses, sight, hearing, taste, and smell, special organs fur which are 

 present in almost all the Vertebrata. 



Reproduction in the Vertebrata is always truly sexual, the sexes 

 are almost always in different individuals, and in no case are com- 

 pound organisms produced by a process of budding or fission. Most 

 are oviparous, producing eggs from which the young are developed. 

 Some retain the eggs within the body till the young are hatched, but 

 no direct connection is established between the young animal and 

 the parent ; and these are sometimes said to be ovo-viviparous. The 

 higher Vertebrates, however, bring forth their young alive, and are 

 said to be riviparous (Latin, vieus, living; and pario, I bring forth). 



Primary Divisions of the Vertebrata. — The Vertebrata are 

 variously divided into great primary sections by different writers, 

 and all of these divisions have more or less merit. Here, however, 

 the classification proposed by Professor Huxley will be followed, and 

 it is not necessary to enter into any consideration of the others. It 

 has also been thought advisable to give in this place a brief account 

 of the leading characters which separate these divisions from one 

 another, though it is not to be expected that the learner will be able 

 to appreciate the full value of these characters till he has completed 

 his study of the Vertebrata as a whole. 



The Vertebrata are divided by Professor Huxley into the following 

 great divisions : — 



I. IcHTi-iropsiDA (Gr. ichthus, a fish ; and opsis, appearance). — In 



