PARASITIC SARCODINA 129 



The life history also varies in the different species, many free- 

 living forms possessing a flagellated stage. On the basis of struc- 

 ture, life history, and habits the old genus Amoeba has been broken 

 into a number of genera distinguishable from each other prin- 

 cipally by the structure of the nucleus and the nature of the cysts. 

 The life history is simple in all of them, so far as it is known. The 

 cases of spore formation, multiple fission, conjugation or sexual 

 processes described by various investigators have not been sub- 

 stantiated by subsequent work. All of the parasitic amebse are 

 characterized by the absence of contractile vacuoles. One species, 

 Endamoeba histolytica, habitually feeds on blood corpuscles and 

 living tissues, and another, Councilmania lafleuri, does so to some 

 extent. The other species apparently feed principally on bacteria 

 and other organic material with which they are associated in their 

 normal habitat. 



The principal amebae occurring in human beings may be ten- 

 tatively grouped in the five genera Endamceba, Councilmania, 

 Endolimax, lodamceba, and Dientamoeba, but the classification 

 of the parasitic amebse cannot yet be considered as satisfactorily 

 settled. The characteristics of these genera are briefly as follows : 



Endamceba; nucleus vesicular with chromatin arranged in a 

 peripheral layer of beadlike granules of fairly uniform size, and a 

 small compact karyosome; a capsule-like structure can usually 

 be seen surrounding the karyosome. Cysts, if produced, with 

 normally 4 or 8 nuclei of similar structure to those of the free 

 forms, and including also glycogen masses and refractile " chro- 

 matoid " bodies, though these masses and bodies commonly 

 disappear before or soon after the cysts become mature. (See 

 Figs. 36 and 37.) 



Councilmania; nucleus with chromatin distributed in a peri- 

 pheral zone on nuclear membrane and in an irregular karyosome. 

 Cysts normally with 8 nuclei which have relatively large and very 

 variable karyosomes, and very little peripheral chromatin. Buds 

 containing single nuclei escape from a pore in the cyst wall until 

 the cyst is emptied of all its nuclei. (See Fig. 41.) 



Endolimax; nucleus vesicular without a distinct peripheral 

 layer of chromatin. A fairly large compact mass of chromatin 

 (karyosome) in the interior, usually more or less eccentric and 

 connected by threads or processes with one or more smaller masses. 

 Mature cysts oval, with four nuclei in the single known species, 

 similar in structure to those of the free forms. The cysts contain, 



