ORGANIZATION OF RHIZOPODA 



161 



superficial clear hyaline layer, the ectoplasm, and a more granular fluid, 

 endoplasm. A pseudopodium may be formed of ectoplasm alone, or it 

 may have a core of endoplasm. Within the endoplasm are to be found 

 the nuclei, food vacuoles, and various granules, while contractile vacuoles 

 are present in the forms which are not parasitic. 



In some Rhizopoda (Foraminifera) the ectoplasm secretes a protective 

 shell known as a theca, which covers the 

 body almost entirely (Fig. 72). A pore is left, 

 through which pseudopodia are protruded, 

 to enable the organism to move about and 

 secure its food. In addition to the main open- 

 ing, the shell may be perforated by niimerous 

 minute pores. Shells of this kind may be 

 formed when the organism is only partially 

 grown, and with increase in size a new and 

 larger shell is made. With further growth 

 others still larger are produced, and these, 

 remaining attached to one another, give rise 

 to many chambered shells, the separate 

 sections of which are variously arranged 

 according to the particular species. The 

 Radiolaria have a perforated membranous 

 central capsule, which divides the cytoplasm 

 into a central mass in which the nucleus 

 lies, and an extracapsular portion or mantle. 

 In the latter siliceous skeletal structures 

 of various kinds are developed. These take 

 the form of shells or spicules, which are often 

 conspicuous for the beauty of their design. 

 Whatever may be the character of the organ- 

 ism, the predominating phase in develop- 

 ment is one which produces pseudopodia, 

 and in the majority no other phase is 

 known to exist. In some, however, a 

 transitory flagellate phase occurs, during 

 which the organism resembles in every 

 respect a member of the class Mastigophora. On this account it is 

 exceedingly difficult to define accurately the limits between the two 

 classes Rhizopoda and Mastigophora. In the latter the flagellate phase 

 is the predominating one, while in the former it is the pseudopodial or 

 amoeboid phase. It has been demonstrated in the case of certain organ- 

 isms (DiniasdtfdiHdha) that the ama'boid or flagellate phase can be pro- 



I. 11 



Fig. 71. — Actinosphceyiuni eich- 

 horni : An Entire Individual 

 ( X 90) AND Portion of An- 

 other ( X 360). (From Lan- 

 kester's Treatise on Zoology, 

 after Leidy, 1879.) 



c.y.i,Contractile vacuole ;c.«.o, position 

 of another contractile vacuole which 

 has just collapsed; cr.fndd vacuole; 

 r., rotifer just engulfed; y/.v , pscudd- 

 Tiodium ; psa.,axis of pscutlupodiuni ; 

 N., nucleus. 



