/. BHIZOPODA: MONERA, LOBOSA. 



189 



definite form, as in the Radiolaria and Heliozoa, the formation of a skele- 

 ton of regular character, as in the Thalamophora, or the development of 

 a peculiar reproduction, as in the Mycetozoa. At the bottom stand tlie 

 Monera and the Lobosa whose characters are mostly negative, for neither 

 form, skeleton, nor reproduction affords systematic distinctions. 



Order I. Monera. 

 The most important character of the Monera is the lack of a nucleus. 

 As with other negative characters this is somewhat uncertain. In many 

 cases, e-specially when the protoplasm is filled with chromatin granules, 

 the nucleus is recognized with difficulty, and hence animals have been 

 described as anucleate in which the nucleus was overlooked. The num- 

 ber of ' Monera ' was formerly very great, but has diminished with the 

 development of microscopic technique. So it is possible, even pirobable, 

 that, in the few forms now remaining in the group, the nucleus has merely 

 escaped observation. On the other hand there are several theoretical 

 reasons which support the idea of anucleate organisms. It is easier to 

 suppose that with the appeai'ance of life there were organisms consisting 

 of but a single substance than that these organisms had nucleus and 

 protoplasm already differentiated. Several species of Protamceba are 

 placed in the Monera. 



Order II. Lobosa (Amcebina). 

 Lobosa are primitive Rliizopoda with 



one or several nuclei. The species of 



Ariueha, forms which owe their name to 



their constant change of shape, arc typical 



(figs. IIG, 110). This change of form is 



due to tlie formation and disappearance 



of a few finger-lilie (lobose) pseudopodia. 



Body and pseudopodia consist of two 



hiyers, a soft granular inner entosarc 



(en) and a firmer, clear, outer ectosarc 



(r^•). In the entosarc is usually a single 



(sometimes several) nucleus {n), which is 



vesicular, and contains either one large 



or several smaller nucleoli. A contractile 



vacuole is usually present. Reproduction 



occurs by division (fig. 110), and iit some 



instances encj'stment has been observed, 



the protoplasm dividing into many hun- 

 dred small amoebfe. 



Most Lobosa occur in fresh water ; the larger forms, like Pelomyxa 



paliistris (2 mm. in diameter) live in the ooze of pools, the smaller, like 



Amoeba proteus and A. princess, on plants or free in the water. The very 

 small A. terricola lives in moist earth. There are also parasites among the 

 Amoebas, like A. coli (0.03 to 0,035 mm. large), rare in colder climates, fre- 



Amceba polmjodia 

 (After F. E. 



Fig. 119.- 

 in division. 



Schulze.) ci\ contractile 

 vacuole ; eh, ectosarc ; 

 entosarc; ?;, nucleus. 



