CHAPTER 19 

 THE GROUPS OF ANIMALS 



In applying the principles of taxonomy systematic workers have 

 often disagreed. This is inevitable because of the many judgments which 

 must be made from meager evidence. When groups of facts seem to 

 point to different conclusions, biologists may and frequently do weigh 

 the conflicting data differently. Various schemes of classification have 

 therefore arisen, all of them agreeing in many major features, differing 

 from one another in less fundamental respects. The one here given may 

 not be the best, but it is in common use. 



The principal groups of animals are given, with brief descriptions 

 and some well-known examples. The definitions are necessarily incom- 

 plete and are often not sufficient to distinguish all the members of one 

 group from those of another. They will serve, however, to give a general 

 concept of classification and a bird's-eye view of the animal kingdom. 



Phylum 1. Protozoa. — These are single-celled animals, mostly of 

 microscopic size, though some are visible to the unaided eye. Some 

 species are colonial, but in these the cells are usually all potentially alike; 

 that is, ^here is no differentiation among the attached cells to form tissues 

 or organs. Protozoa live in very varied situations but usually require 

 moisture. Many of them live in the soil. They are exceedingly com- 

 mon in ponds, streams, lakes, and oceans and may be attached to solid 

 objects, be buried in mud or debris, or swim freely in the water. Many 

 of them are parasitic in other animals. Some of the parasitic ones cause 

 disease, as malaria, dysentery, and African sleeping sickness in man. 

 Some protozoa live in other animals in a relation that is beneficial to the 

 host as well as to the guests. A most remarkable example of mutual 

 benefit is that received and conferred by certain protozoa in the digestive 

 tracts of termites. These insects, whose food is wood, would be quite 

 unable to digest the cellulose without the aid of the guest protozoa. 



Untold numbers of protozoa live in the sea, and lived there ages ago. 

 The great limestone beds, chalk cliffs, and quartzite and flint deposits 

 are made up of shells of ancient protozoa. Noctiluca is a marine proto- 

 zoon which is responsible for some of the remarkable phosphorescence 

 observable in disturbeji waters at night. 



There are three principal modes of locomotion. Some protozoa 

 thrust out pseudopodia, projections of their protoplasm, and then flow 



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