76 Royal Institution : — 



are composed, not being invested (any more than the sarcode of 

 the body) by any limiting membrane, coalesce with each other and 

 with it ; and thus the particle which has been entrapped becomes 

 actually imbedded in the gelatinous mass, and gradually passes 

 towards the central part of it, where its digestible portion undergoes 

 solution, the superficial part of the body with its pseudopodial 

 prolongations in the meantime recovering its previous condition. 

 Any indigestible portion, as the shell of an Entomostracan, or the 

 hard case of a Rotifer, finds its way to the surface of the body, and 

 is extruded from it by a process exactly the converse of that by 

 which it was drawn in. 



If, now, it be asked, in what consists the peculiar animality of 

 beings thus destitute of every feature that we are accustomed to 

 associate with the idea of an animal, — that is, if it be inquired 

 what are the characters by which they are distinguished from 

 vegetable organisms of equal simplicity, — the physiologist cannot 

 with confidence reply that sufficient evidence is afforded by the 

 movements of the Amoeba and Actinophrys ; since among the lowest 

 Plants there are many, which, at least in certain stages of their 

 lives, are endowed with yet even greater activity. A more positive 

 and satisfactory distinction lies in the nature of their aliment, and 

 in the method of its introduction. For whilst the protophyte obtains 

 the materials of its nutrition from the air and moisture that surround 

 it, and possesses the power of detaching oxygen, hydrogen, carbon 

 and nitrogen from their previous binary compounds, and of uniting 

 them into ternary and quaternary organic compounds (chlorophyll, 

 starch, albumen, &c), the simplest protozoon, in common with the 

 highest members of the animal kingdom, seems utterly destitute 

 of any such power, and depends for its support upon organic sub- 

 stances previously elaborated by other living beings. Further, 

 whilst the protophyte obtains its nutriment by simple imbibition, 

 the protozoon, though destitute of any proper stomach, extemporizes, 

 as it were, a stomach for itself in the substance of its body, into 

 which it ingests the solid particles that constitute its food, and 

 within which it subjects them to a regular process of digestion. 

 Hence these simplest members of the two kingdoms, which can 

 scarcely be distinguished from each other by any structural cha- 

 racters, seem to be physiologically separable by the mode in which 

 they perform those actions wherein their life most essentially consists. 



There are found, both in fresh and salt waters, numerous examples 

 of this Rhizopod type, which do not present any essential advance 

 upon the Amoeba and Actinophrys ; and a large proportion of these 

 are endowed with a shelly investment which may be either calcareous 

 or siliceous, — the former being the characteristic of the Foraminifera, 

 the latter of the Polycystina. In some of these testaceous forms, 

 the pseudopodia are put forth only from the mouth of the shell, 

 whilst in other cases this is perforated with minute apertures for 

 their passage ; but where there are no such apertures, the sarcode 

 body not unfrequently extends itself over the entire external surface 

 of the shell, and may give off pseudopodia in every direction. 



