182 



ZOOLOGY 



bilaterally symmetrical animals consisting of many 

 cells, without true segmentation and without any trace 

 of a notochord, and without the special characters of 

 the mollusks. The fact is that the' bilaterally sym- 

 metrical type of animals gave rise to a great many 

 independent branches, some of which assumed great 

 importance ; while others, though very distinct in struc- 

 ture, remained relatively insignificant. 



Phylum Arthropoda (page 257) 



Arthropods The jointed-footed animals, such as the insects, centi- 

 pedes, crabs, spiders, etc. 



Phylum Mollusca (page 243) 



Mollusks The mollusks, including snails, slugs, clams, cuttle- 



fish, etc. Although mollusks, annelids, and bryozoans 

 are so different in appearance when adult, they show 

 curious resemblances in the early stages. 



Phylum Prochordata (page 320) 



The forms which, while lacking a vertebral column, 

 nevertheless breathe by means of gill slits, and have at 

 least in some stages a more or less developed notochord. 

 A miscellaneous group, unsatisfactory because its divi- 

 sions are so little related to one another. It is sometimes 

 included with the vertebrates as a phylum Chordata. 



Phylum Vertebrata (page 328) 



Vertebrates The vertebrates ; fishes, reptiles, amphibians, birds, 

 and mammals, including man. The relationship be- 

 tween the various phyla may be roughly indicated as 

 follows, the most primitive types being placed lowest in 

 the diagram : 



Prochor- 



dates 



