THE SARCODINA 87 



form of .granules throughout the entire cell, as in some of the Masti- 

 gophora ; again, it is confined to a solid sphere without membrane or 

 intra-nuclear vacuoles ; or, there may be a membrane and a single 

 compact mass of chromatin which occupies the centre of the distinct 

 nucleus, and is separated from the membrane by hyaline matter. In 

 other cases, there may be two or more karyosomcs or chromatin 

 reservoirs, or there may be a great number of granules in the nucleus 

 without the reservoirs (Amoeba proteus). In some of the Rhizopoda 

 (Euglypha) and Heliozoa (Actinophrys and Actinosplicerium), the 

 nucleus is strikingly similar to that of metazoan cells, consisting of 

 chromatin in the form of a reticulum and a network of linin (Figs. 

 14 and 54). 



The number of nuclei is also quite variable, many forms having 

 only one (Amoeba proteus, Actinophrys, etc.) ; others, two (Amoeba bi- 

 nucleata, Arcella, etc.); while some have many, the giant Amoeba, 

 Pelomyxa, having, according to Bourne ('91), about ten thousand, 

 although, even with this large number, the proportion of nuclear 

 substance to the total mass of the organism is about the same as in 

 other cells (Bourne). In almost all of the shelled forms, a multiple 

 number of nuclei is the rule, but in the many-chambered Reticulari- 

 ida, every chamber does not possess a nucleus, the number of 

 nuclei being smaller than the number of chambers, thus indicating that 

 these forms are not colonies, but syncytia, or multinucleate cells. 



D. The Contractile Vacuole 



Contractile vacuoles are almost entirely absent in the marine forms 

 (Reticulariida, Radiolaria), and in a few of the fresh-water forms 

 of Sarcodina (Protamoeba, Pelomyxa, Myxodictyum, Protogencs, etc.), 

 but they are generally present in the Amoebida and Heliozoa, some- 

 times two or three in one organism. The number of contractile 

 vacuoles is quite variable. In most of the naked forms there is 

 but one; this, however, maybe of large size, sometimes measuring 

 one-quarter of the volume of the organism (Actinophrys, Actino- 

 splicerium). In the shelled forms, on the other hand, there are 

 two or more (2-3 in Euglypha, 12 or more in Arcella). 



The position of the vacuole in the naked forms is also variable, 

 but becomes fixed in the shelled forms and in the Heliozoa. In the 

 shelled forms they sometimes lie in the middle zone about the edge 

 of the granular region (Euglypha), sometimes around the periphery of 

 the flattened body, while in other forms they are found now in one 

 zone, now in another. In all cases, shortly before contraction, they 

 come to lie close to the outer edge, and in some cases they form 

 minute wart-like excrescences. 



