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rather than of structure. To make the minor groups, however, 

 structural resemblances of an easily accessible nature would next 

 have to be resorted to, as shape of bills and feet; number of 

 feathers in tail and wings; form, color, and even size, might also 

 be pressed into service. In such classifications affinity becomes 

 guesswork, and true relationships are thrown to the winds. 



The simplest and most lowly organized of all known animal 

 forms are those designated by biologists as the Protozoa. For 

 the most part they are composed of single cells of living matter, 

 and are elementary in all particulars. Multi-celled animals, or, 

 as they have been called, the Metazoa, is a division created to 

 contain all other animals. Spanning these two primary groups, 

 we have certain protozoans in which simple cells are massed to- 

 gether in globular form. 



The next most general morphological fact to be observed, and 

 which can be practically utilized in a taxonomic way, is that the 

 Metazoa are susceptible of division into two primary groups, on 

 the basis that in one set we find the animals possessed of a cen- 

 tral axis oi- cord, composed of either jointed osseous segments or 

 vertebra^; of cartilage, or simply of the notochord; while in the 

 remaining set nothing of the kind exists. In other words, we 

 have the Invertebrata and the Tcrtcbrata. But the latter possess 

 also a spinal chord and brain, hence they may likewise be desig- 

 nated as the Chordataj the invertebrate group, wherein the 

 nerve-cords are either ventral or lateral, are known as the Won- 

 chordata. Other good distinctions also exist. 



Here again, however, when we get down toward the simplest 

 types, we meet witli forms that approach with remarkable near- 

 ness the lowest ones of the alternate series. That is, in these 

 cases certain invertebrates approach the Vertebrata in certain 

 features, and, vice versa, we meet at the very starting point of the 

 vertebrate series with most puzzling animals, as, for example, the 

 Tunicata; the worm-like Balanoglossus and Cephalodisous, and the 

 famous Lancelet (Amphiosous) . 



The Protozoa have been well classified into their main groups 

 and subdivisions by biologists; so, too, the invertebrate Meta- 

 zoa have been similarly dealt with, and we meet therein with 

 more or less natural divisions created to contain the Sponges 

 (Porifera) ; the jelly-fish, hydroids, and their allies (Goelenterata) : 

 the Worms (Yermes); the starfish, etc. (Eclrinodermata) ; the 

 shelled animals (Mollusca), and other groups for crustaceans, 



