Page:Natural History Review (1861).djvu/477

Rh and their outlines present a sharpness which indicates that the substance of which their exterior is composed possesses considerable tenacity. No movement of granules can be seen to take place along the surface of the pseudopodia; and when two of these organs come into contact, they scarcely show any disposition even to mutual cohesion, still less to a fusion of their substance. Sometimes the protrusion seems to be formed by the ectosarc alone, but more commonly the endosarc also passes into it, and an active current of granules maybe seen to pass from what was previously the centre of the body, into the protruded portion, when the latter is undergoing rapid elongation whilst a like current may set towards the centre of the body from some other protrusion which is being withdrawn into it. It is in this manner that an Amœba moves from place to place; a protrusion like the finger of a glove being first formed, into which the substance of the body itself is gradually transferred; and another protrusion being put forth, either in the same or in some different direction, so soon as this transference has been accomplished, or even before it is complete. The kind of progression thus executed by an Amœba is described by most observers as a "rolling" movement, this being certainly the aspect which it commonly seems to present; but it is maintained by MM. Claparède and Lachmann that the appearance of rolling is an optical illusion, for that the nucleus and contractile vesicle always maintain the same position relatively to the rest of the body, and that "creeping," or reptation, would be a truer description of their mode of movement. On this view, these animals have their ventral constantly differentiated from their dorsal surface, it being from the former alone that the pseudopodian extensions proceed; and thus a transition would seem to be indicated towards the testaceous Amœbina (Arcella, Difflugia, &c.) in which the dorsal surface is invested by a shell, and the pseudopodia are strictly limited to the ventral region. It is in the course of its movement from place to place, that the Amœba encounters particles which are fitted to afford it nourishment; and it appears to receive such particles into its interior through any part of the ectosarc, whether of the body itself or of any of its lobose expansions, insoluble particles which resist the digestive process being got rid of in the like primitive fashion.

The Amœban, like the Actinophryan, type shows itself in the testaceous as well as in the naked form; and it is of importance to notice, that whilst the "test" of Arcella is formed by a membranous (probably chitinous) exudation from the animal itself, that of Difflugia is chiefly made up of grains of sand, fragments of shell, or other foreign particles, cemented together. The resemblance of the animals of these two genera is so close, that no systematist has ever proposed to separate them by more than a generic distinction; and if the dissimilarity of the material of their "test" be not admitted as a differential character of grave importance, I can see no reason for attaching more weight to the distinction between the chitinous of Gromia and the calcareous shell of the ordinary Foraminifera,