SPECIAL ZOOLOGY. 



Since comparative anatomy and the tlieor}' of evolution have 

 made their impression upon systematic zoology one recognizes in 

 classification not only a means of arranging the species, but also 

 the jiossibility of expressing the relations which the larger and 

 smaller groups bear to each other. The solution of these prob- 

 lems demands an accixrate knowledge of comparative 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 prob- 

 lems are not all equally advanced towards solution. 



In general it may be said that certain natural grou^js are 

 recognized: (1) Chordata; (2) Mollusca (after the elimination 

 of the Brachiopoda) ; (3) Arthropoda; (4) Echinoderma: (5) 

 Coelenterata (after the separation of sponges) ; (6) Protozoa. On the 

 other hand, it is j'et uncertain exactly how to regard the worms, 

 brachiopods, polyzoa, and a few other forms. The general ten- 

 dency is to distribute the worms into at least three branches (flat 

 w^orms, round worms, and annelids) and to unite the Polvzoa and 

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

 in species and of little importance in a general account of the ani- 

 mal kingdom are placed on the same basis as the large and exceed- 

 ingly important groups of vertebrates, arthropods, and molluscs, 

 and thus obtain, especially in the eyes of the beginner, an impor- 

 tance which does not belong to them. It therefore seems better in 

 an elementary work to pursue a more conservative course. 



183 



