204 VERTEBRATE ANIMALS. 



to this alone ; but in the lower Verfebrata it is discharged in- 

 to the hinder extremity of the alimentary canal, and is evacu- 

 ated along with the undigested portions of the food. 



The nervous system varies greatly in its development in 

 the Vertebrata. In the little fish called the Lancelet, the main 

 mass of the nervous system consists of a cord of nervous mat- 

 ter, representing the spinal marrow, but not having in front 

 any enlargement which represents the brain. In all the other 

 Vertebrata the central masses of the nervous system (termed 

 the cerebrospinal axis) consist of a nervous cord (the spinal 

 cord) contained in the canal formed by the neural arches of 

 the vertebrae, and of an anterior mass of nervous matter, which 

 is protected by the skull, and is termed the encephalon or 

 brain. The size and development, 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," which are connected with the 

 special senses, sight, hearing, taste, and smell, special organs 

 for which are present in almost all the Vertebrata. 



Reproduction in the Vertebrata is always truly sexual, the 

 sexes are always in different individuals, and in no case are 

 compound organisms produced by a process of budding or fis- 

 sion. Most are oviparous, producing eggs from which the 

 young are developed. Some retain the eggs within the body 

 till the young are ready to be hatched, and these are some- 

 times said to be ovo-viviparous. The higher Vertebrates, 

 however, bring forth their young alive, and are said to be 

 viviparous (Latin, vivus, living ; and pario, I bring forth). 



Peimaey Divisioiirs of the Veetebeata. — The Verte- 

 brata are variously divided into great primary sections by dif- 

 ferent writers, and all of these divisions have more or less 

 merit. Here, however, the classification proposed bj' Prof. 

 Huxley will be followed, and it is not necessary to enter into 

 any consideration of the others. It has also been thought ad- 

 visable 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 com- 

 pleted his study of the Vertebrata as a whole. 



The Vertebrata are divided by Prof. Huxley into the fol- 

 lowing great divisions : 



I. IcHTHTOPSiDA (Gr. ichthus, a fish; and opsis, appear- 

 ance). — In this section are included the fishes (Class Pisces), 



