COMPARISON OF ANIMALS WITH ONE ANOTHER.
This head is also comprised within that of our entire Cyclopmdia. The glance we shall cast over the field it embraces will, therefore, be very cursory, and the views taken of the objects it presents extremely general.
_Physical qualities and material constitution of animals —In point of size, animals differ most widely from one another. The existence of some is only made known by the aid of a powerful microscope, the length of others ex ceeds a hundred feet, and their weight amounts to many tons. These extremes include animals of every intermediate bulk.
Theibrin assumed by animals presents many more interesting particulars for study and in vestigation than the mere bulk of their bodies. The consideration of this accident has even been made the ground of a classification of the objects included within the animal kingdom by several naturalists, and although not adopted as the sole basis of any one now generally received, it nevertheless furnishes the element upon which several of the classes even of the most recent are established. Some animals present themselves in the likeness of a globule, others of a filament, and others of a small flattened membrane (the cyclides). Various animals, again, from exhibiting no uniform or regular shape, have been entitled amorphous or beteramorphous.
Animals which exhibit a determinate form naturally arrange themselves into two classes ; their bodies are either disposed around a centre, or they consist of two similar halves cohering along a middle plane or axis ; the first are the radiata, the second the binaria or symmetrica of naturalists. The radiata are not a very extensive class of animals, neither is their organization extremely complicated. The symmetrical is a much more numerous class than the radiated, and includes within its limits creatures of such simple structure as the en tozoa, and of such complicated fabric as quad rupeds and man. Of the symmetrical animals, some consist of a mere trunk without appen dices or limbs ; those that are provided with limbs, again, have them in the shape of feet, fins, wings, or hands, according to the media in which they live. In some the body forms as it were a single piece, in others it is divided into portions, such as head, trunk, and tail. Sometimes it is naked ; at others it is covered with shells, scales, spines, hair, &c. Some times the general integument is continuous, unpierced by any opening that leads to the interior, at others it is reflected inwards, and lines extensive cavities there contained.
within, and of the phenomena that occur without them. Even this distinction, how ever, is only applicable as regards species con siderably raised above the lowest ; would we indicate the differences between the most in ferior members of either series we must con • descend upon particulars, and, in some in stances, even call in analogy and inference to our aid in laying down the chart of their re semblances and dissimilarities.
With regard to structure, as may be imagined, the amorphous tribes, at the bottom of the scale, are the most simple of all. The bodies of some of these are without any internal cavity, and without any division of parts; they are homogeneous masses, generally gela tinous in appearance, and simply cellular in structure, without arrangement into tissues or particular organs. The external surface of these animals imbibes the matters which are fitted to subserve the purposes of nutrition, and we may presume that it throws off by transpiration such particles as are worn out or have accomplished this end. The external surface is also the organ of respiration in these animals. They procreate by the evolution of gemmi from their surface, and if they possess sensibility the element to which it is attached must be generally diffused throughout their sub stance.
The organization of the radiata becomes con siderably more complicated. Fluids are no longer absorbed from the external surface of the body; we meet with an internal cavity, the rudiment of a digestive apparatus, having a single opening in some of the species, which serves consequently for both mouth and anus, but in others presenting two openings, a mouth properly so called on one side of the body, and an anus on the other. Through the walls of this cavity the nutritive fluids make their way, and infiltrate the general mass of the animal's body. In this class we also discover the rudiments of a nervous and of a muscular sys tem. The nervous system consists of rounded masses of a soft whitish substance, equal in number to that of the radii composing the animal, connected together by slender white cords, and sending off filaments of the same description to all parts of the body, but espe cially to the outer integument, and to the inter nal digestive apparatus. The muscular system consists of reddish and whitish fasciculated fibres disposed in the line of the motions. The external surface of these animals is still the only organ of respiration they possess.