1 2 INTKOUUCTION. 



known as the " sympathetic " system of nerves. Further, in the 

 YertAinita there is alw.iys an i/itermd skeleton, the central stem 

 of which is usually constitvited by a true backbone or "vertebral 

 column." Whin this is not present, there is a structure which will 

 be afterwards (lfscril>e(l as the " notochord " or "chorda Jorsalis." 

 Lastly, the Innln of Vertebrate animals, when present, are never 

 more than four in nundjer, and they are always turned away 

 from tliat side nf liie l)ody on whicli the main masses of the nervous 

 system are plarcd. 



The subdivision of the Invertebrate animals is divided into five 

 great gnjujjs or " morphological types," which are known as sub- 

 kiiig<loin!<.^ These jirimary tyjjes of structure are known to natu- 

 ralists under the technical names of the Protozoa, Ctelenterata, Ecliin- 

 odermaUi, A iiiiiihixn, and JlliiUiixtri. Their distinguishing characters 

 and more important subdivisions will be spoken of hereafter, and 

 need not, tlierefore, be noticed here. The division of the Verte- 

 brate animals constitutes by itself a sixth .sub-kingdom, which is 

 simply known under the name i>f Vcrti'linita. There are, therefore, 

 si.r ]nimary ty]ies nf structure or sub-kingdoms, to one or other of 

 which all known animals may be referred. The highest of these 

 forms alone the division of the " Vertebrate Animals," whilst the 

 five lower sub-kingiloms are grouped together under the collective 

 riame of " Invertelirate Animals." 



Each siili-ii/nfi/iiin, therefore, comprises a group of animals which 

 are constructed ujion a common plan or typie, however gi'eatly this 

 ty|)e may be nioditieil ; and each is, in turn, broken up into s\d)- 

 divisions which are termed " /■/ii.<xfv." Each " class," again, is divided 

 into " y/-' /■';•.<," each " oiiler " into '\fiimih'es" each "family" into 

 " i/eiicrii" and each "genus" into ^' sprcii's." To dejine the term 

 "species," which is the smallest detinite division accepted by 

 naturalists, is, in the light of recent researches, no easy matter. 

 AVe may, lioAvever, consider that eacli " species," or, as we should 

 commonly say, "kind," of animals cou.sists of such individuals as 

 resemble one another in all the essential characters of their struc- 

 ture, and are able, directly or indirectly, to prodvice other fertile 

 indiviiluals like themselves. Even in the above definiti(m, we nnist 

 recollect that the individuals of a species are not ^jrccf'scVy alike, and 

 it commonly happens that a number of the individuals of a species 

 have .some s[]ecial |)cculiaiity or ])cculiarities, by which they can 

 readily be .separated from the rest, When this is the case, the 

 individuals thus distinguish.able constitute what is called ■.i^'oirictj/," 



' M;niy iiatuiinists n-g.'inl lln' Spoutri's as foriiiini; a sixth ilivisioii, to wliioli 

 till- iiaiiii- .4' /',,!■ fn-fi is oltcii pivcu ; luit tliis division is lianlly i'c[uivalcut in 

 rani< to ii siili kinLnloni, 



