214 THE PROTOZOA 



are visible to the naked eye, and some of the Radiolaria, Foramin- 

 ifera, and Mycetozoa, attain to a size that must be considered 

 gigantic for Protozoa. The more primitive forms, on the other 

 hand, are often very minute. 



The body-form is of two principal types, related to-distinct habits 

 of life namely, the amoeboid type, characteristic of forms that 

 creep on a firm substratum ; and the radiate type, seen in floating 

 forms. Amoeboid forms are found aquatic, semiterrestrial, and 

 parasitij ; radiate forms are for the most part pelagic, living floating 

 or suspended in large masses of water, marine or fresh-water. 



The protoplasmic body is in many cases distinctly differentiated 

 into clear motile ectoplasm and granular trophic endoplasm. The 

 surface of the protoplasm is naked, or may be covered in rare in- 

 stances (Amoeba verrucosa, A. terricola, etc.) by a very thin pellicle 

 which modifies, but does not restrain, the amoeboid movements. 

 A resistant cuticle or cell-membrane investing the body is not 

 formed, but an external shell or internal supporting skeleton is 

 frequently present. 



The locomotor organs in the adult are always pseudopodia, which 

 may be of various types lobose, filose, or reticulose (Chapter V., 

 p. 46) ; they may lie in one plane, as in creeping forms, or may be 

 given off on all sides, as in pelagic forms. The youngest forms 

 (swarm-spores) paa,y be flagellate or amoeboid. In some cases the 

 pseudopodia of the young forms may differ markedly in character 

 from those of the adult ; for example, the adult Amoeba proteus has 

 fluid protoplasm with thick lobose pseudopodia, but the young 

 amoebula produced from the cyst of this species has viscid proto- 

 plasm with sharp, spiky pseudopodia (Scheel). 



The free-living Sarcodina are almost without exception holozoic, 

 capturing other organisms by means of their pseudopodia, and 

 devouring them; but the remarkable genus CJdamydomyxa (p. 243) 

 has chromatophores, and can live in either a holozoic or holophytic 

 manner, like some flagellates ; and the genus Paulindla, allied to 

 Euglypha, also possesses chromatophores. and is capable of holo- 

 phytic nutrition (Lauterborn). 



The nuclear apparatus consists of one or more nuclei, in addition 

 to which chromidia may be present. A single nucleus is charac- 

 teristic of the majority of species, even of many which grow to 

 very large size, such as many Radiolaria, in which the nucleus also 

 attains to proportions relatively gigantic. In other cases increase 

 in the size of the body is accompanied by multiplication of the 

 nuclei ; there may be two nuclei constantly, as in Amoeba biniicleata 

 (Schaudinn), or several, as in Difflugia urceolata, or many hundreds, 

 as in Actinosphosrium and Pelomyxa, or even thousands, as in the 

 Mycetozoa. In such forms the adult is a plasmodium, but the 



