96 ELEMENTS OF BIOLOGY 



sub-kingdoms, the single cell forms, or protozoa, and the many cell 

 forms, or metazoa. The sub-kingdoms are further divided on the 

 basis of somewhat closer resemblances, into other groups termed 

 PHYLA. Thus, for example, all worm-like animals that consist of a 

 series of rings, have this and several other grosser features in com- 

 mon and are regarded as related; all animals which conform to 

 these characteristics are placed in the phylum Annelida. The num- 

 ber of phyla that compose the Animal kingdom is frequently given 

 as thirteen, although some authorities divide some of these and 

 recognize more. 



Within the phylum there may be many types which resemble 

 each other only in having the diagnostic characters of the phylum. 

 So the phylum may be composed of a number of classes. Simi- 

 larly, the members of a class are not identical, but are divisible 

 into ORDERS. Orders are further separated into families and families 

 into GENERA. Within the genus a further grouping is made on the 

 basis of very close resemblances and somewhat minor differences. 

 These ultimate groups are termed species and within the species 

 only a very close examination reveals individual differences. It is 

 also often convenient to make further sub-divisions of each group, 

 as SUB-KINGDOM, suB-CLAss, SUB-GENUS, and SO ou. In passing from 

 the broader to the more narrow groups, from the kingdom to the 

 species, dissimilarities become less and less, while similarities be- 

 come more and more numerous and extensive, until within the 

 species the various individuals differ in exceedingly minor charac- 

 ters. Expressed as a contracting series of relationships, one may with 

 some accuracy think of members within the Animal kingdom as 

 being related only through some common ancestor in a very remote 

 past; in a similar way the members of a phylum may be thought 

 of as being somewhat more closely related to each other than to 

 members of other phyla; within the species the relationship is real 

 and demonstrable, for various members of the species may in reality 



