The physical structure of a vast number of animals has been investigated from such as are a mere microscopic speck to the highest grade of animal life; but very little is comparatively known of their intelligence and means of communication. We know not by what means a pointer and greyhound make an agreement to hunt together; nor how each dog is not only aware that his companion possesses a property which he has not, but that by their united talents they might accomplish their purpose, which is merely sport, for they never eat the game.[2] The undulations of the air and water are no doubt the means by which most animals communicate; but there is reason to believe that many inhabitants of the earth, air, and water are endowed with senses which we do not possess, and which we are consequently incapable of comprehending. SECTION II. PROTOZOA. THE Protozoa are the very lowest forms of animal existence, the beginning and dawn of living things. They first appear as minute shapeless particles of semi-fluid sarcode moving on the surface of the waters. The pseudopodia, or false feet, with which they move, are merely lobes of their own substance which they project and retract. In creatures of a somewhat higher grade the form is definite, the pseudopodia, numerous and filamental, serving for locomotion and catching prey; and from the resemblance they bear to the slender roots of plants are called Rhizopods.[3] The microscopic organisms possessing these means of locomotion and supply, are of incalculable multitudes, and of innumerable forms. Thus the waters, as of old, still ‘bring forth abundantly the moving creature that hath life;’ in them the lowest types of the two great kingdoms have their origin, yet they are diverse in the manifestation of the living principle, that slender but decided line which separates the vegetable from the animal Amœba. CLASS I.—RHIZOPODA. The Amœba, which is the simplest of the group, is merely a mass of semi-fluid jelly, ‘changing itself into a greater variety of forms than the fabled Proteus, laying hold of its food without members, swallowing it without a mouth, digesting it without a stomach, appropriating its nutritious material without absorbent vessels or a circulating system, moving from place to place without muscles, feeling (if it has any power to do so) without nerves, multiplying itself without eggs, and not only this, but in many instances forming shelly coverings of a symmetry and complexity not surpassed by those of any testaceous animal.’ [Illustration: Fig. 86. Amœba princeps.]