SPECIAL ZOOLOGY. 



SINCE comparative anatomy and the theory of evolution have made 

 their impress upon systematic zoology one recognizes in classification 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 knowledge of compara- 

 tive anatomy and embryology and a complete knowledge of animal forms 

 based upon them. We are yet far from such a knowledge, farther with 

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

 problems are not all equally advanced towards solution. 



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

 (i) Chordata; (2) Mollusca (after the elimination of the Brachiopoda); 

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

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

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

 The general tendency is to distribute the worms into at least three branches 

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

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

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

 are placed on the same basis as the large and exceedingly important 

 groups of vertebrates, arthropods, and molluscs, and thus obtain, 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 may be seen by a sharp eye 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 by milli- 

 meters, especially where hundreds of individuals 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 they 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 directly by the 

 protoplasm. 



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