182 THE INDUCTIONS OF BIOLOGY. 



logically central though often not centrally situated^, a general 

 mass of protoplasm surrounding it, and an inclosing differ- 

 entiated portion in contact with the environment. These essen- 

 tial elements are severally subject to various complications. 



In some simple types the limiting layer or cortical sub- 

 stance can scarcely be said to exist as a separate element. 

 The exoplasm, distinguished from the endoplasm by absence 

 or paucity of granules, is continually changing places with it 

 by the sending out of pseudopodia which are presently drawn 

 back into the general mass : the inner and outer, being un- 

 settled in position, are not permanently differentiated. Then 

 we have types, exemplified by Lithamoeha, constituted of 

 protoplasm covered by a distinct pellicle, which in sundry 

 groups becomes an outer shell of various structure: now 

 jelly-like, now of cellulose, now siliceous or calcareous. 

 While here this envelope has a single opening, there it is 

 perforated all over — a fenestrated shell. In some cases an 

 external layer is formed of agglutinated sand-particles; in 

 others of imbricated plates, as in Coccospheres ; and in many 

 others radiating spicules stand out on all sides. Through- 

 out sundry classes the exoplasm develops cilia, by the wavings 

 of which the creatures are propelled through the water — 

 cilia which may be either general or local. And then this 

 cortical layer, instead of being spherical or spheroidal, may 

 become piano-spiral, cyclical, crosier-shaped, and often many- 

 chambered: whence there is a transition to colonies. 



Meanwhile the inclosed protoplasm, at first little more 

 than a network or foamwork containing granules and made 

 irregular by objects drawn in as nutriment, becomes vari- 

 ously complicated. In some low types its continuity is 

 broken by motionless, vacant spaces, but in higher types 

 there are contractile vacuoles slowly pulsing, and, as we may 

 suppose, moving the contained liquid hither and thither; 

 while there are types having many passive vacuoles along 

 with a few active ones. In some varieties the protruded 

 parts, or pseudopodia, into which the protoplasm continually 



