86 THE ANATOMY OF INVERTEBRATED ANIMALS. 



face, along with the Radiolaria, all over the globe (see p. 80). 

 The late investigations of Archer and others have demon- 

 strated the existence of a considerable number of fresh-water 

 Radiolaria. 



Extensive masses of tertiary rock, such as that which is 

 found at Oran, and that which occurs at Bissex Hill, in Bar- 

 badoes, are very largely made up of exquisitely preserved 

 skeletons of Radiolaria. But, though there can be little 

 doubt that Radiolaria abounded in the Cretaceous sea, none 

 are found in the chalk, their silicious skeletons having prob- 

 ably been dissolved and redeposited as flint. 



2. The Pkotoplasta. — The proper Amoebm have broad 

 and ovate pseudopodia, and resemble Protamoeba (p. 75) very 

 closely; but they present an advance upon its structure, by 

 exhibiting a distinct endoplast (nucleus) and a contractile 

 vacuole. In Arcella, there are many such nuclei. They thus 

 stand in somewhat the same relation to Protamoeba as Acti- 

 nophrys does to Protogenes. 



Moreover, there are Amoebm in which the power of throw- 

 ing out pseudopodia is confined to one region of the body ; 

 and others, as Arcella, in w T hich a shell is formed over the 

 rest of the body. In other Amoebm, as A. radiosa> the pseu- 

 dopodia are few, narrow, and but little mobile. But the 

 Amoebm present no such diversity of skeletal development as 

 the Foraminifera do. They multiply by division, and in 

 some cases — e. g., A. sphmrococcus of Haeckel — become en- 

 cysted before they divide. 



Amoebm (the " proteus animalcules " of the older writers) 

 are not uncommon, and sometimes are very abundant, in 

 fresh waters ; they also occur in damp earth and in the sea, 

 but there is much doubt whether many of them are to be 

 regarded as independent organisms, or whether they are not 

 rather stages in the development of other animals or even 

 of plants, such as Myxomycetes. Leaving out the contractile 

 vacuole, the resemblance of an Amoeba in its structure, man- 

 ner of moving, and even of feeding, to a colorless corpuscle 

 of the blood of one of the higher animals is particularly note- 

 worthy. 1 



3. The Gregaeotd^e are very closely allied to the Amm- 

 bm, but, in the cycle of forms through which they pass, they 

 curiously resemble Myxastrum. In form they are spheroidal 



1 Contractile vacuoles have been observed in the colorless blood-corpus- 

 cles of Amphibia under certain conditions. 



