220 THE PROTOZOA 



At the present time the life-history has been worked out satisfac- 

 torily in but f ew free-living amoebae, but in such protean organisms 

 it is quite unsafe to attempt to characterize or define a species 

 without a knowledge of the whole life-cycle. As regards the 

 familiar Amoeba proteus, for example, practically all that is known of 

 its life-cycle is that it encysts and multiplies within the cyst to 

 form a great number of small amcebulae, very different in appear- 

 ance from the parent-organism ; the amoebulse creep out of the cyst, 

 and probably grow up into the adult form (Scheel). Calkins 

 adduces arguments in favour of the occurrence of a sexual cycle, 

 which remains at present, however, purely conjectural. 



The majority of free-living amoebae are aquatic in habitat. A 

 certain number, however, are semi-terrestrial, inhabiting damp 

 earth, moss, etc. Such is Amoeba terricola (vide Grosse-Allermann). 

 The " earth-amoebae," like other terricolous Protozoa, probably 

 play a great part in keeping down the numbers of the bacteria and 

 other organisms in the soil, and thereby lessening its fertility from 

 an agricultural standpoint (compare Russell and Hutchmsoii, 24 ; 

 Goodey, 16). 



A great many species of amoebae are found living within the 

 bodies of animals of all kinds, for the most part in the digestive 

 tract. The entozoic amoabae are commonly placed in a distinct 

 genus, Entamoeba, distinguished from the free-living forms by little, 

 however, except their habitat and the general (but not invariable) 

 absence of a contractile vacuole. A common example is Entamoeba 

 blattce, from the intestine of the common cockroach ; others are 

 E. ranarum of the frog (Dobell, 236, 237) ; E. muris of the mouse 

 (Wenyon) ; the species parasitic in the human intestine, presently 

 to be mentioned ; E. buccalis (Prowazek), from the human mouth ; 

 and many others. Chatton has described a species, Amoeba mu- 

 cicola, ectoparasitic on the gills of Labridce, and extremely patho- 

 genic to its host. 



Life-History. So far as it is possible to generalize from the scanty 

 data available at present, the development of many free-living 

 species of amoebae appears to be of a type very similar to that of 

 Arcella, described in a previous chapter (p. 179). In the free state 

 the organisms reproduce themselves in two ways : first, " vegeta- 

 tively, 5 ' by simple binary fission, preceded by a division of the 

 nucleus, which varies in different cases from a pro mitosis (p. 109) 

 of the simplest type to very perfect mitosis ; secondly, by forma- 

 tion of chromidia and subsequently of secondary nuclei, round 

 which the cytoplasm becomes concentrated to form, a number of 

 internal buds, destined to be set free as amcebulae, agametes, which 

 grow up into the adult form. In addition to these two methods of 

 reproduction in the free state, the animal may become encysted, 



