THE LOBOSA 77 



the individual retained by the old shell is or is not capable 

 of further reproduction, but the occurrence of an immense number 

 of empty shells in cultures of Centropyxis and its allies suggests that 

 it may die after one act of fission. The individual that has formed 

 a new and larger shell, however, certainly divides again, giving rise 

 by a similar process to a daughter individual with a still larger 

 shell. When by these processes of fission the full size is reached, 

 the nucleus degenerates, after giving rise to an expanded chromidial 

 network which, with about two-thirds of the protoplasm, protrudes 

 from the mouth of the shell, is pinched off, and escapes. The 

 remaining one-third of the protoplasm and the degenerate nucleus 

 that remain in the shell probably die. 



The escaped protoplasm may give rise to one of two broods of 

 gametes. In one brood (the megagametes) the chromidia give rise 

 to a nucleus and the protoplasm forms a shell ; in the other, after 

 a nucleus is formed from the chromidia and a shell is formed as 

 in the first brood, a division into four individuals (the microgametes) 

 takes place, and each of these escapes and forms a small shell. 

 Conjugation takes place between the larger and smaller individual 

 gametes, and the zygote escapes to form a new shell like that of 

 the parent. 



ORDER Gymnamoebida, 



The surface of the body either naked or provided with a 

 thin flexible membrane through which the pseudopodia can be 

 protruded. 



Genera Amoeba. The generic name Amoeba is often applied 

 to any naked amoeboid organism without reference to its subsequent 

 or antecedent history. As our knowledge of the natural history 

 of the simpler Protozoa widens it becomes more evident that the 

 generic name should be used only in a restricted sense. The limits- 

 we place upon the use of the generic name can only be regarded 

 as provisional. Further investigations may well prove that the 

 species now included in the genus Amoeba, ought to be still further 

 separated into subgeneric or generic groups. 



The characters of the genus may be summarised as follows : 

 Solitary Gymnamoebida, with a few short blunt pseudopodia, a 

 single contractile vacuole, and one or more nuclei. No membrane 

 covering the body in the trophic phase of life. Freshwater or 

 marine. 



Nine or ten distinct species have been described from fresh 

 water in this country (Cash). They are usually found in the mud 

 at the bottom of ponds or creeping on submerged vegetation. 

 Some of the rarer forms are found in Sphagnum bogs. One of the 

 commonest species is Amoeba proteus (Fig. 12, 5), a species capable 

 of considerable variation in form, but usually exhibiting several 



