34 INVERTEBRATE ANIMALS. 
simplest possible manner and with the simplest possible appara- 
tus. There is no permanent mouth, no stomach or alimentary 
canal of any kind, no respiratory or excretory organs, and even 
no distinct aperture for the extrusion of indigestible food. The 
only distinct structure which is at all concerned in nutrition is 
a rudimentary contractile cavity, the first foreshadowing of the 
heart in the higher animals. As regards the functions of rela- 
tion, it is questionable how far the Amq@éa can be said to have 
distinct perceptions or sensations of any kind. It has no ner- 
vous system or organs of sight or hearing, and in all proba- 
bility it has nothing more than a general sensibility to light. 
It appears, however, to be fully aware when any object comes 
in contact with a pseudopodium, and even to have some idea 
whether this is fit for food. Locomotion, as we have seen, is 
entirely effected by the temporary processes of sarcode or pseu- 
dopodia, and there are no permanent organs set aside either for 
‘locomotion or for prehension—that is, for seizing external objects. 
The only other member of the Amm@da which deserves notice 
is the Diffiugia (fig. 3, 6), which is not uncommonly found in 
fresh water. Diffugia in its essential structure does not differ 
from the Amada, but the greater part of the body is enclosed 
in a sort of case or carapace, mostly composed of grains of 
sand, within which the animal can retire completely. The cara-' 
pace is open at one end, and the pseudopodia are protruded 
from this aperture. The animal generally creeps about head- 
downwards, so to speak—that is to say, with the closed end of the 
carapace elevated above the surface on which it is moving. 
ORDER III. FoRAMINIFERA.—The next order of the R/izzopoda 
is that of the Poraminifera, comprising animals which at first 
sight appear to be highly complex, but which are really much 
less highly organised than the Amaba. The Foraminifera may 
be defined as Rhizopoda in which the body ts protected by a 
shell or “test” there is no nucleus or contractile vesicle» and 
the pseudopodia are extremely long and thread-like, and interlace 
with one another so as to form a network. 
The most obvious and striking character of the Foraminifera 
is the possession of an outer case or shell, and for a long time 
they were known to naturalists by their shells alone. As the 
shell or test is usually very beautiful and often very complex, 
the Foraminifera were consequently placed at. first amongst the 
true shell-fish (AZod/usca), very much in advance of their true 
