DESCRIPTIONS OF PREPARATIONS. 



and collapses suddenly. A new vacuole is formed at or near the spot 

 occupied by its predecessor. There is rarely more than one of these pulsa- 

 tile spaces present. 



The animal when in a state of repose or after being disturbed forms a 

 spherical or oval ball, about T |^ of an inch in size. It sometimes occurs in 

 this condition surrounded by a delicate membrane forming a * hypnocyst.' 

 It is then 'resting' owing to drought or plentiful nutrition. When it 

 passes into a state of active motion, the surface of the body is covered with 

 numerous clear protrusions of ectosarc. Certain of these elongate into 

 pseudopodia while the remainder are withdrawn, and at the same time the 

 animal begins to flatten out. The endosarc flows into the growing pseudo- 

 podia, which at first extend in various and opposite directions. But sooner 

 or later one or two elongate in a given direction, and the rest retract, and 

 the animal moves in a determinate course. It is now, according to the 

 phase of shape assumed by the pseudopodia, either ramose, as in Fig. 

 13, C., dendroid, or palmate. When it floats freely suspended in water the 

 pseudopodia extend in all directions, giving it a radiate or stellate appear- 

 ance. The pseudopodia attain in their growth characters which are distinc- 

 tive of the species or the group to which these Rhizopodan organisms 

 belong. In Amoeba Proteus they are finger-like, i.e. digitate, simple (C :/.) 

 or branched (C : />') Their tips are either blunt or tapering. Elongation 

 takes place by an onward flow of ectosarc followed by the extrusion into 

 the ectosarc of a current of endosarc. The flow is, as a rule, not even but 

 more or less sudden, as though there were a surface-resistance to be over- 

 come. The posterior part of the body, where the protoplasm is receding, 

 has often a lobed or mulberry-like appearance, as shown in Fig. 13, C. 

 Similar lobes may be observed on a pseudopodium during its retraction. 

 Any single lobe in the posterior mass might commence to elongate, and 

 grow into a pseudopodium : and the direction of the animal's movement, 

 indicated in C. by the arrows, might be thus reversed. In the extended 

 condition this species may measure the ^ of an inch. 



An Amoeba has been observed dividing into two. The protoplasmic 

 bridge or filament connecting the two halves, gradually becomes more and 

 more delicate and lengthened, and then finally snaps. It is uncertain 

 whether or no Amoeba Proteus ever forms spores in the shape of minute 



Amoeba or Amoebidae, such as have been observed in Pelomyxa palustris. 



\ 



Protozoa, Ray Lankester, Encyclopaedia Brit. (ed. ix.) xix. 1885. 



Infusoria ( = Ciliata\ Saville Kent, Manual of the Infusoria, 2 vols., and 

 Plates, London, 1880-82. For Paramecium, see.vol. ii. p. 483 ; PL xxvi. Figs. 28-30. 



Amoeba. Leidy, Fresh-water Rhizopoda of North America, United States 

 Geological Survey, xii. 1879. For A. Proteus, see p. 31 ; Pis. i; ii; iv, Fig. 25 ; vii, 

 Figs. 13-19; viii, Figs. 17-30. Of. Gruber, Z.W.Z. xlL 1885. 



