CHAPTER II. 



THE PEOTOZOA. 



In its feeblest manifestations, the contractility of animals 

 results in mere changes of the form of the body, as in the 

 adult Gregarinm / but, from the sluggish shortenings and 

 lengthenings of the different diameters of the body which 

 these creatures exhibit, all gradations are traceable, through 

 those animals which push out and retract broad lobular pro- 

 cesses, to those in which the contractile prolongations take 

 the form of long and slender filaments. Whether thick or 

 filamentous, such contractile processes are called "pseudo- 

 podia," when their movements are slow, irregular, and in- 

 definite ; " cilia " or " flagella," when they are rapid and occur 

 rhythmically in a definite direction ; but the two kinds of or- 

 gans are essentially of the same nature. It will be convenient 

 to distinguish those Protozoa which possess pseudopodia, as 

 myxopods, and those which are provided with cilia or flagella, 

 as mastigopods. 



The Protozoa are divisible into a lower and a higher 

 group. In the former — the Monera — no definite structure is 

 discernible in the protoplasm of the body ; in the latter — the 

 Exdoplastica — a certain portion of this substance (the so- 

 called nucleus) is distinguishable from the rest; 1 and, very 

 commonly, one or more " contractile vacuoles " are present. 

 The name of contractile vacuoles is given to spaces in the pro- 

 toplasm, which slowly become filled with a clear, watery fluid, 

 and, when they have attained a certain size, are suddenly 

 obliterated by the coming together, on all sides, of the proto- 

 plasm in which they lie. This systolic and diastolic move- 

 ment usually occurs at a fixed point in the protoplasm, at regu- 

 lar intervals, or rhythmically. But the vacuole has no proper 



1 I adopt this distinction as a matter of temporary convenience, though 

 I entertain great doubt whether it will stand the test of further investigation. 



