46 ZOOLOGY sect. 



is predominant, although the amceboid and encysted conditions 

 frequently occur. 



Class 4. Sporozoa. — Parasitic Protozoa without special loco- 

 motive parts in the adult. Encystation is almost universal, and 

 the young may be flagellate or amceboid. 



Class 5. Infusoria. — Protozoa which are always ciliated, either 

 throughout life or in the young condition. 



CLASS I.— RHIZOPODA. 



1. Example of the Class — Amoeba proteus. 



Amoeba has been fully described in the preceding chapter ; it 

 will therefore be unnecessary to do more than recapitulate the 

 most essential features in its organisation. 



It is an irregular mass of protoplasm (Fig. 30, e) about ^ mm. 

 in diameter, produced into irregular processes or pseudopods (psd) 

 of variable size and form and capable of being protruded and 

 retracted, often with considerable rapidity. The protoplasm is 

 divisible into a granular internal substance or endosarc and a clear 

 outer layer or ectosarc ; the difference between the two is hardly a 

 structural one, but depends simply on the accumulation of granules 

 in the central portion. The granules are, for the most part, various 

 products of metabolism — proteinaceous or fatty. 



Imbedded in the endosarc is a large nucleus (mi), of spherical form, 

 consisting of a clear achromatic substance, enclosed in a membrane, 

 and containing minute granules of chromatin. The contractile 

 vacuole (c. vac), a very characteristic structure of the Protozoa, lies 

 in the outer layer of the endosarc, and exhibits rhythmical move- 

 ments, contracting and expanding at more or less regular intervals. 



Amoeba feeds by ingesting minute organisms (Fig. 30, c,f.,vac.) 

 or fragments of organisms — i.e., by enveloping them in its substance, 

 retaining them until the proteids they contain are dissolved and 

 assimilated, and then crawling away and leaving the undigested 

 remnants behind. 



Amoebae are sometimes found to undergo encystation; the 

 pseudopods are withdrawn and the protoplasm surrounds itself 

 with a cell-wall or cyst (d, cy), from which, after a period of rest, 

 it emerges and resumes active life. The cyst is formed of a 

 chitinoid material — i.e., a nitrogenous substance allied in composi- 

 tion to horn and to the chitin of which the armour of Insects, 

 ^Crayfishes, etc., is composed. 



Reproduction takes place by simple or binary fission ; direct or 

 amitotic division of the nucleus is followed by division into two of 

 the cell-body (i). Occasionally two Amcebse have been observed to 



