SPECIAL ZOOLOGY. 



Since comparative anatomy and the theory of e\olution have made 

 their impress upon systematic zoologv one reco,L;nizes in chissilication not 

 only a means of arranging the species, but also the possibility of expressing 

 the relations which the larger and smaller groups bear to each other. The 

 solution of these problems demands an accurate knowleilge of compara- 

 tive anatomy and eml)ryology and a complete knowledge of animal forms 

 based upon them, ^^'e are yet far from such a knowledge, farther with 

 regard to some groups than others, aiid as a consequence systematic 

 problems are not all ec[ually advanced towards solution. 



In general it may Ije said that certain natural groups are recognized: 

 (i) Cliordata; (2) jNIollusca (after the elimination of the Bracliiopoda) ; 

 (3) Arthropoda; (4) Echinoderma; (5) Cadenterata (after the separation 

 of sponges) ; (6) Protozoa. On the other hand, it is yet uncertain exactly 

 how to regard the worms, brachiopods, polvzoa, and a few other forms. 

 The general tendency is todistriliute the worms into at least three branches 

 (fiat worms, round worms, and annelids) and to unite the Polyzoa and 

 Brachiopoda in a branch of JMoUuscoida. In this way groups poor in 

 species and of little importance in a general account of the animal kindom 

 are placed on the same Iiasis as the large and exceedinglv important 

 groups of verteljrates, arthropods, and molluscs, and thus olUain, espe- 

 cially in the eyes of the beginner, an importance which does not belong to 

 them. It therefore seems better in an elementary work to pursue a rather 

 conservative course. 



PHYLUM I— PROTOZOA. 



All of the Protozoa are small; some mav lie seen bv a sharp eve as mere 

 specks, but the majority are so minute as to be invisible except with a micro- 

 scope. On the other hand, a few have a diameter to be measured ]i\ milli- 

 meters, especially where hundreds of indixiduals are united in colonies. 

 This small size is a result of the fact that the Protozoa are single-celled 

 animals. 'Like all cells they consist of protoplasm, and thev have the 

 further cell attribute, one or more nuclei. Being unicellular, it follows 

 that they lack true tissues and true organs; alimentary canal, nervous 

 system, sexual organs, etc. The functions of nourishment, sensation, 

 movement, and reproduction are performed more or less directlv by the 

 protoplasm. 



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