AMOEBINA. 899 



throughout its whole substance, and it may or may not show a hyaline 

 edge at the spot which is extending onwards in locomotion. But with an 

 increase in density, the granules, &c., are confined to the more central 

 portion and leave a more or less pronounced hyaline border. Hence a 

 distinction into an outer ectosarc and an inner endosarc 1 . In the Testacea 

 the granules are frequently aggregated in a middle zone. Their proto- 

 plasm also, when the test is rigid, does not under ordinary circumstances 

 fill it, but a connection is maintained between the two by contractile 

 threads which retract the animal when it is molested. The configuration 

 of the pseudopodia appears to depend on the density of the protoplasm. In 

 the Testacea they are emitted only at the aperture or in amphistomatous 

 genera at both apertures, from a mass of clearer protoplasm, and they are 

 either few and digitiform, occasionally containing granules and sometimes 

 slightly branched, e. g. in Arcella, Difflugia, &c., or they are filose, i. e. 

 filamentous, and generally somewhat numerous, simple or branched, 

 rarely widely extended as *in Pamphagus, and seldom undergoing anas- 

 tomosis 2 . In the Nuda they are subject to much variation. When the 

 protoplasm is fluid it not infrequently flows as a whole in one direction ; 

 or the pseudopodia are broad, more or less irregular lobes, seldom, as in 

 Amoeba Proteus ; digitiform, and the granules and other contents of the 

 protoplasm flow into them. A steady onward flow is also observable in 

 the locomotion of species with very dense protoplasm and a well-defined 

 ectosarc, e. g. Hyalodiscus with the ectosarc as a border on the advancing 

 side ; Amoeba verrucosa and A. terricola with an uneven advancing edge 

 or knob-like projections and wrinkled surface. So too in A. tentaculata 

 and A. actinophora with coagulated surface, in Zonomyxa and Tricho- 

 sphaerium with differentiated cuticular structures (supra}. But the four 

 last-named have also special pseudopodia, purely ectosarcal except in 

 Zonomyxa, of no use however as organs of locomotion, but serving solely 

 as nutritional, and perhaps also as tactile organs. In A. tentaculata they 

 protrude from the ends of conular eminences, are short, pointed, and dis- 

 play curving motions ; they are retracted in locomotion or persist tentacle- 

 like on the advancing margin. They are short and digitiform in A. actino- 

 phora, and put forth at one spot or near together ; hence the Amoeba has 

 the look of a Cochliopodium. In Tricho sphaerium they are elongated, cylin- 

 drical, and issue from gaps in the cuticle ; in Zonomyxa branched and 

 stretching the cuticle wherever they protrude until it becomes indistinguish- 



1 Gruber insists strongly on the fact that there is but one kind of 'protoplasm in an Amoeba, and 

 that when the granules, &c. either do not follow at once the forward movement of the body, or are 

 confined to a central part, the cause is not the existence of different layers, but the greater density 

 of the protoplasm. See Z. W. Z. xli. pp. 196, 201 ; Biol. Centralblatt. vi. p. 5. 



2 Amphitrema stenosloma is said by Niisslin to emit both digitiform and filose pseudopodia, 

 one at one end of the test, the other at the other end, simultaneously. The digitiform alone are used 

 in locomotion. Z. W. Z. xl. pp. 718-9. 



3 M 2 



