1 -2 INTRODUCTION. 



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

 Vertebrata there is always an ■internal skeleton, the central stem 

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

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

 be afterwards described as the " notochord " or " chorda dorsalis." 

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

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

 from that side of the body on which the main masses of the nervous 

 system are placed. 



The subdivision of the Invertebrate animals is divided into five 

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

 kiiigdoms.^ These primary types of structure are known to natu- 

 ralists under the technical names of the Protozoa, Ccelenterata, Echin- 

 odermata, Anmdosa, and Mollusca. Their distinguishing characters 

 and more important subdivisions will be spoken of hereafter, and 

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

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

 simply known under the name of Vertebrata. There are, therefore, 

 six primary types of 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-kingdoms are grouped together under the collective 

 name of " Invertebrate Animals.'' 



Each sub-kingdom, therefore, comprises a, group of animals which 

 are constructed upon a common plan or type, however greatly this 

 type may be modified ; and each is, in turn, broken up into sub- 

 divisions which are termed " classes." Each " class," again, is divided 

 into ''orders," each "order" into ''families" each "family" into 

 "genera" and each "genus'' into "species." To define the term 

 " species,'' which is the smallest definite division accepted by 

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

 ' We may, however, consider that each " species," or, as we should 

 I commonly say, " kind," of animals consists of such individuals as 

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

 i ture, and are able, directly or indirectly, to produce other fertile 

 individuals like themselves. Even in the above definition, we mvist 

 recollect that the individuals of a species are not precisely alike, and 

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

 have some special peculiarity or peouliaiities, by which they can 

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

 individuals thus distinguishable constitute what is called a " variety," 



1 Many naturalists regard the Sponges as forming a sixth division, to which 

 the name of Porifera is often given ; hut this division is hardly equivalent in 

 rank to a siih-kingdom. 



