/. RHIZOPODA: 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 the 

 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, especially 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 probable, 

 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 appearance of life there were organisms consisting 

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

 protoplasm already differentiated. Several species of Protamcsba are 

 placed in the Monera. 



Order II. Lobosa (Amoebina). 

 Lobosa are primitive Ehizopoda with 

 one or several nuclei. The species of 

 Amoeba, forms which owe their name to 

 their constant change of shape, are typical 

 (figs. 116, 119). This change of form is 

 due to the formation and disappearance 

 of a few finger-like (lobose) pseudopodia. 

 Body and pseudopodia consist of two 

 layers, a soft granular inner entosarc 

 (en) and a firmer, clear, outer ectosarc 

 (ek). In the entosarc is usually a single 

 (sometimes several) nucleus (ri), which is 

 vesicular, and contains either one large 

 or several smaller nucleoli. A contractile 

 vacuole is usually present. Reproduction 

 occurs by division (fig. 119), and in some 

 instances encystment has been observed, F VJ- 

 the protoplasm dividing into manv him- Bchnlae.) cv, x contractile 



J vacuole ; ek, ectosarc ; en, 



dred small amoebae. entosarc; w, nucleus. 



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

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

 Amceba proteus and A. princeps, on plants or free in the water. The very 

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

 Amceba3, like A. coli (0.02 to 0.035 mm. large), rare in colder climates, fre- 



