226 VKliTKl'.RATE ANIMALS. 



their must important function, as tlie retention of tliis substance 

 within tlie lioily rapidly causes deatli. The secretion of the kidneys 

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

 this alone ; but in the liAvi'r ycrti'hnitji it is discharged into the 

 hinder extremity of tlie alimentary canal, and is evacuated along 

 with the undigested portions of the fond. 



The urrvoiis si/strui varies greatly in its development in the Verte- 

 hriita. In the little fish called the lyancelet, the main mass of the 

 nervous system i.-onsists of a cord of nervous matter, representing the 

 .spinal marrow, but ni>t having in front any enlargement which repre- 

 sents the brain. In all the other Vt'rtehrala the central masses of 

 the nervous system (termed the fi'rehro-sjiinol n.rix) consist of a nerv- 

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

 neural arches of the vertebra', and of an anterior mass of nervous 

 matter, wliich is j)rotected by the skull, and i.s termed the "enceph- 

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

 vary enormously in dift'erent Vertelirates ; and in the hjwer 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 for which are 

 present in almost all the Vertehrn'd. 



Reproduction in the Vertchrotn is always truly se.xual, the sexes 

 are almost always in ditferent imlividuals, and in no case are com- 

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

 are oviparon.i, jiroducing eggs from which the young are developied. 

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

 no direct connection is estalilished between the young animal and 

 the parent ; and these are sometimes said to be oro-viei.parous. The 

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

 said to lie ciripuriins (Latin, ricux, living; and purirt^ I bring forth). 



Prim.vry I)ivisioxs of Tine Vertebrata. — The Vi'iichmtn are 

 va.rio\isly divided into great prnuary sections by ilill'erent writer.s, 

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

 the classification proposed by Profes.sor 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 phice a brief account 

 of the leading clianictei's which separ.-ite these divisions from one 

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

 to a]ipreciate the full value of these ch.aracters till he has completed 

 Ins study of the Vi'Hrhriiln as a whole. 



The Vi'r/rtirata are diviiled by Professor Huxley into the following 

 great divisions : — 



I. IciiTin'oPSiDA (i)r. iclitliits, a fish ; and ttpxix, appearance). — lu 



