CLASSIFICATION OF ANIMALS. 23 



organized body of similar cells. They are the lowest of animals, 

 are found only in water and moist places, and are usually invisible 

 to the unaided eye. 



34. Key for the rough identification of the animals the student is 

 most likely to find. 



A. Microscopic, or at least minute; chiefly aquatic. 



I. Unicellular, or if of several cells these are loosely asso- 

 ciated Protozoa. 



II. Multicellular, with definite organs; the individual cells 

 not easily distinguished 



1. Body not composed of a series of segments, moves 



with a gliding motion by means of cilia. 



"Flat-worms." 



2. Body showing evidences of being made up of segments. 

 Segments obscure; a disc of cilia about the mouth, 



giving the appearance of a rotating wheel. 



Rotifers (Wheel- worms). 

 Segments manifest: two or more clusters of hair-like 



appendages on each segment .Annulata (aquatic). 

 Segments variable: numerous paired appendages 



jointed Crustacea (Water-fleas) . 



B. Not microscopic. 



I. Purely aquatic; chiefly marine. 



Not symmetrical, growing massive, attached. .Sponges. 

 Radially symmetrical the repeated structures arranged 



radially. 



Attached and often colonial . . . Ca-lenterates (Polyps). 



Free-swimming; jelly-like . . . .Calenterates (Jelly-fish). 



Free, but sluggish; skeleton and spines (commonly). 



Echinoderms. 



II. Marine, fresh water, and terrestrial. 



Without conspicuous paired appendages (including fins, 

 legs, wings, etc.). 



With protecting shells Mollusks (See p. 227). 



One valve or piece ; usually spiral. 



Snails, Periwinkles, etc. 



Two valves to the shell . . . Clams, Mussels, Oysters. 

 Without protecting shells ....Worms. (See p. 204.) 

 (Here might be found also some insect larvae). 

 Not strictly aquatic, but in moist places. 



Annulata (Earthworms). 



