THE PEOTOZOA (continued) 



SECTION D. THE LOBOSA l 



GYMNOMYXA (Homokaryota), with lobate or pointed unbranched 

 pseud opodia without an axis and with one or more definite nuclei. 



In a large number of the characteristic genera of Lobosa the 

 body consists of a small plastid of protoplasm protruding a few 

 lobate pseudopodia by means of which a slow progression is 

 effected, and exhibiting one nucleus and a contractile vacuole. 



In addition to these characteristic forms, however, other genera 

 must be included in the same class in Avhich the body is protected 

 by membranous or rigid shells (Thecamoebida), with radiating and 

 pointed pseudopodia (Trichosphaerium, etc.), with two (Arcella), or 

 numerous nuclei (Pelomyxa), and with no contractile vacuole 

 (Endamoeba, etc.). 



In many Lobosa, such as Amoeba terricola (Penard [20]) and 

 others, the superficial protoplasm secretes a membranous envelope 

 through which the pseudopodia may be protruded or particles of 

 food ingested. In Trichosphaerium the envelope is relatively 

 thick, gelatinous in texture, and provided with a series of very 

 delicate radiating spicules, mainly composed of carbonate of 

 magnesia. Spicules similar to these are also found in the ecto- 

 plasm of Amoeba pilosa (Cash), in which no true membrane is 

 formed. In Dinamoeba (Leidy) the spicules occur in a hyaline 

 jelly that surrounds the body. 



In the Thecamoebida a definite shell is formed through which 

 the pseudopodia cannot penetrate. In this case the pseudopodia 

 can protrude only through a definite and permanent mouth or 

 pore in the shell, which it is convenient to call the pylome (Hartog). 



In some forms of Amoeba and in other genera there is 

 often seen an apparent differentiation of the protoplasm into a 

 clear outer layer, called the ectoplasm, and a more granular and 

 more fluid central substance called the endoplasm. This appear- 

 ance is more clearly defined when the protoplasm is very active 

 and several pseudopodia are protruded. In the quiescent stages 

 and conditions of life the ectoplasm usually disappears or becomes 

 extremely attenuated, and in species or forms with only one or two 



1 By Prof. S. J. Hickson, M.A., F.R.S. 

 68 



