92 Biology in America 



sionally they form one or more rings about the body, while 

 again they are condensed into several long flexible processes 

 by means of which the animal crawls about over the weeds 

 or bottom. Still others possess long slender spines, which may 

 either have no apparent use, or may serve as springing organs, 

 the animal lying still for a time and then suddenly starting 

 to roll and tumble about as though possessed of a very devil 

 of unrest. 



Yet another great group of Protozoa, the flagellates, derive 

 their name from the flagella or whip-like processes by means 

 of which they swim. Most Protozoa are actively motile, but 

 some are attached by stalks, either singly or in branching 

 groups. These are sometimes fixed, and sometimes furnished 

 with delicate muscle fibrils, which contract suddenly when the 

 animals are disturbed. 



Usually naked, some Protozoa are enclosed in cases or shells. 

 One lives in a tube, with a lid which closes as the animal 

 retracts, and opens as it expands. Many of these shells are 

 of great beauty and complexity. The infusorian Coleps bears 

 a shell comprised of numerous plates arranged in circles 

 around the body, the flagellate Peridinium has a delicately 

 sculptured shell of twenty-one plates and Ceratium is en- 

 closed in a shell bearing long, horn-like processes. But the 

 most remarkable development of shells is found in the 

 Foraminifera and Kadiolaria, whose remains ferm so large 

 a part of the ooze covering the bottom of the sea, and which 

 have produced valuable deposits of building stone and chalk 

 in the past. 



Many Protozoa have developed primitive organs of diges- 

 tion, excretion and respiration, while some have contractile 

 fibrils in the outer layer of the body, which serve as primitive 

 muscles. Yet others have the suggestion of eyes in the form 

 of pigment spots, which doubtless are responsive to light. 



In these early differentiations of structure we have forecast 

 for us the conditions in the Metazoa or many-celled animals 

 with their special organs and corresponding ''division of 

 labor " or work which these organs have to do. 



Not alone in structure are many Protozoa highly specialized. 

 In manner of life they vary widely. Many of them are 

 exclusively marine, others inhabit only fresh waters, while 

 still others may be found in fresh and brackish water alike. 

 Mostly free living, a few have developed the habit of com- 

 mensalism, or close association with some other organism. 

 The ciliates, Trichodina and Kerona, are usually found gliding 

 over the surface of the fresh water polyp Hydra. The Radio- 

 laria harbor symbiotic algae, by means of which the synthesis 

 or construction of carbohydrates is possible, after the manner 



