PEOTOZOA. 1 9 



that tlie protoplasm becomes converted into an aggregate of cells. 

 There is no definite internal cavity (" body-cavity "), and no nervotts 

 system is developed. Definite digestive organs are wanting, or present 

 only in rudimentary form. 



If we except the Sponges — which many naturahsts regard as a 

 separate division — the Protozoa do not rise, morphologically, above 

 the condition of which is called a " cell." That is to say, a typical 

 Protozoan consists of a mass of protoplasm, surrounded by a thickened 

 layer or " wall," and enclosing a central solid or vesicular body or 

 " nucleus," often containing in its interior, or attached to its exterior, 

 a still smaller body or " nucleolus." The outer layer of the proto- 

 plasm may, however, remain unchanged, not becoming differentiated 

 into a " wall," and the " nucleus '' and " nucleolus " may be wanting. 

 In such cases, the organism constitutes what is usually called a 

 " cytode," rather than a. " cell." 



On the other hand, in the Sponges the organism becomes " multi- 

 cellular," the primitive protoplasm becoming ultimately divided 

 into a larger or smaller number of separate " cells." For this 

 reason it is not unusual to associate the sponges with the whole 

 series of animals higher than the Protozoa, and to speak of them 

 collectively as the Metazoa. 



The substance of which the body of any Protozoiin is composed is 

 of the nature of protoplasm, usiially very slightly differentiated or 

 altered in any way, and commonly known as " sarcode " (Gr. sar.r, 

 flesh ; eidos, form). Viewed under the microscope, the sarcode 

 appears as a transparent, colourless, or variously coloured viscid sub- 

 stance, exhibiting numerous minute darker particles or granules 

 scattered through it, these being probably of a fatty nature. Along 

 with these dark granules and molecules there can generally be ob- 

 served a larger or smaller number of variously sized round clear 

 spaces, which are known by the name of " vacuoles." Some of these 

 " vacuoles " seem to be constantly present ; but many are of mere 

 temporary occurrence, and these seem to be merely little drops of 

 water included in the soft protoplasm, and sometimes sun-ounding a 

 particle of food. Often one of the vacuoles is not only permanent 

 and fixed in position, but is much larger than the rest, and is " con- 

 tractile," having the power of dilating and contracting at definite 

 intervals. This is known as the " contractile vesicle " (fig. 3, A), 

 and may, perhaps, be looked upon as a rudimentary organ of circu- 

 lation. Sometimes there are two or more contractile vesicles. 



Very commonly the external layer of sarcode composing the body 

 of a Protozoan is comparatively dense and free from granules, while 

 the internal protoplasm is softer and more fluid, and is charged with 

 •rranules and vacuoles. Thus the protoplasm may be distinguished 



