CLASSIFICATION OF ANIMALS 177 



ture and stages in development, should be examined. Distinguishing 

 features are: bilateral symmetry; metamerism; three germ layers; a 

 coelom; a dorsal, tubular, central nervous system formed by infolding, 

 as in the frog embryo; a notochord at some stage in the individual's 

 existence, for example, in the immature stages only (frog) or 

 throughout life (amphioxus) ; gill slits present in all embryos and in 

 some adults. 



(q) Other Phyla. — In addition to the types examined in para- 

 graphs (a) to (p) several small groups of uncertain relationships are 

 recognized and sometimes classed as phyla. If group names not in- 

 cluded in the foregoing study are noted in Fig. 74, or otherwise, de- 

 termine the organization of such forms and relate them to one or 

 another of the principal phyla to which they might be appended 

 instead of being classed as separate phyla. 



Exercise 2. — Grouping of Phyla. 



(r) With the foregoing review of representative types from the 

 various phyla as a foundation, you can now examine the characteris- 

 tics of the phyla with a view to arranging them into larger divisions 

 on the basis of broad structural differences, such as the single-celled 

 or many-celled condition; the presence or absence of a gut cavity; the 

 diploblastic or triploblastic body-plan with its two or three germ 

 layers; the presence or absence of a coelom; and a metameric or non- 

 metameric organization. Examine carefully Figs. 75 and 76. Under- 

 stand how such an arrangement as that shown in Fig. 75 sug- 

 gests evolutionary relationships. What do you understand to be 

 the meaning of natural classification? What is the significance of 

 structural similarities among animals? 



Exercise 3. — The Use of a Key. 



(s) Animals belonging to any phylum are grouped into smaller 

 subdivisions on the basis of structural characteristics. The following 

 key will enable you to understand the method by which the classifica- 

 tion of any particular animal within the phylum can be determined. 

 The material indicated is readily obtainable in most localities. For 

 more extensive practice, use should be made of more detailed keys such 

 as may be found in standard textbooks dealing chiefly with classifica- 

 tion. 



(t) Key to classes of Arthropoda.^ 



1 From A. S. Pearse and F. G. Hall, "Laboratory Manual for General Zoology," 

 University of Wisconsin, 1922. 



