• INTRODUCTION 23 



species may be defined as a group of interbreeding individuals 

 which, while they may differ markedly among themselves, yet 

 resemble each other more closely than they do those of any other 

 group; the characters that distinguish the group being consid- 

 erable, not obliterated by intermediate forms, and inherited from 

 generation to generation." 



8. The Principal Phyla of the Animal Kingdom 



The principal phyla of the animal kingdom as outlined in the 

 following paragraphs are presented in this place since they will 

 be of value for reference purposes during the perusal of the more 

 detailed accounts in the succeeding chapters. The numbers 

 after each phylum indicate approximately the number of living 

 species known at the present time. 1 The groups of animals of 

 more or less uncertain systematic position have been omitted 

 from this outline (see Chap. IX). 



(1) Protozoa. — Single-celled animals; often colonial; sperm 

 and egg cells usually wanting: 8500. 



(2) Porifera. — Sponges. Diploblastic (?) ; radially symmet- 

 rical, number of antimeres variable ; body-wall permeated by 

 many pores and usually supported by a skeleton of spicules or 

 spongin. 2500. 



(3) Coelenterata. — Jellyfishes, Polyps, and Corals. Diplo- 

 blastic; radially symmetrical, with usually four or six anti- 

 meres; single gastro- vascular cavity; no anus; body- wall con- 

 tains peculiar structures known as nematocysts or stinging cells 

 4200. 



(4) Ctenophora. — Sea Walnuts or Comb Jellies. Triplo- 

 blastic; radial combined with bilateral symmetry; eight radially 

 arranged rows of paddle plates. 100. 



(5) Platyhelminthes. — Flatworms. Triploblastic; bilaterally 

 symmetrical; single gastro- vascular cavity; no anus; presence 

 of coelom doubtful. 4600. 



l I am indebted to Professor Henry S. Pratt for the numbers given. 



