Sigh school zoology. 253 



Sarcodina these are of the nature of " pseudopodia," t.e, less 

 permanent processes of the sarcode or cell-plasma than the 

 flagella or cilia, and either thread-like or lobate in form. 



14. In passing from the study of the lower to the higher 

 Protozoa we shall have an opportunity of seeing to what extent 

 " organization " can be carried within the limits of a single cell. 

 The Sarcodina offer compai'atively little of such differentiation ; 

 food-particles absorbed by the pseudopodia are conveyed into 

 the softer cell-plasma at any point of the surface, and the undi- 

 gested remains are similarly thrust out anywhere. Reproduc- 

 tion is effected chiefly by division into two or many cells, in 

 some, a resting stage of " encystment," preceding division. 



15. Among the simplest is the genus Amoeba (Fig. 189), 

 called so on account of its ceaseless change of form, the endo- 

 plasm, which contains the nucleus, is more diffluent than the 

 ectoplasm which forms the pseudopodia and contains the " con- 

 tractile vacuole," a structure which appears to discharge the 

 function of an excretory organ in these and other Protozoa. 

 The Sarcodina ai-e not all naked like the Amceba ; in the 

 order to which it belongs (Rhizopoda, — called so on account 

 of the root-like branches of the pseudopodia in many), 

 most of the forms secrete shells (Thalamophora), which may 

 be perforated all over for the escape of thread-like pseu- 

 dopodia, or have merely one aperture through which similar 

 processes escape or lobate ones like the Amoeba's. The latter 

 is the case in Arcella and Difflugia (Fig. 189, 2 and 3), while 

 in Euglypha and others the proces.ses are thread-like 'Fig. 189, 4), 

 These are fresh-water forms, in which the shells are formed of 

 chitinous matter, or of foreign particles cemented together, but the 

 marine forms — generally called Foraminifera — have for the most 

 part a calcareous shell. This may be imperforate or perforate, and 

 is generally composed of numerous chambers disposed in variously 

 formed spirals (Fig. 190). Although of small size, these Foramini 



