That faculty which the two portions of the earthworm's body possess of manifesting the vital properties independently of one ano ther, and even after having been separated, may be explained to a certain degree by the known structure of these animals and the general laws of physiology. With the exception indeed of the generative organs which are concentrated in a peculiar part of the body, it is easy to observe that each seg ment of the body is almost the exact repe tition of all the rest : they all possess the same organs, and, however the total number of rings may vary, there results no change of any im portance in the general structure of the animal. Now it may be laid down as a law in phy siology, that a parity of organization neces sitates a similitude of action ; and it results that as in depriving an earthworm of a given number of segments no organ is removed of which it does not still retain the analogue, no function is completely destroyed ; and conse quently that if such a mutilation should weaken the vital action, it does not change its nature. This holds good for both the segments of the animal : each continues to possess all the organs essential to individual existence, and consequently if their resisting energy be suf ficiently great, there can be no reason why they should not continue to live independently of one another, and become two distinct worms.* But if the anterior moiety thus becomes a perfect animal, it is probable that this may not happen to the posterior portion, but that the new individual formed by this part will always continue deprived of generative organs. For the anterior moiety retains exclusively the reproductive organs of the original individual, and there is nothing which authorizes the belief that the earthworm possesses the power from being simply mutilated, of reproducing the whole apparatus on any part of the posterior moiety. This, however, is a circumstance which it would be easy to determine.
From the sketch that we have given of the organization of the annelida, it will be seen that there exists in this branch of zoology many hiatuses. Anatomists, in fact, have hardly paid attention to any but the leech, the earth worm, and the naffs, and we possess only a vague notion of the internal structure and physiology of the erratic and tubicolar species; their comparative study would form an interest ing subject of research.
ANUS, (in anatomy,) from Anus vel An nus, a round, a circle, (syn. ostium recti, podex,. culus. Gr. Trecovroc. Fr. anus. Germ. After. Ital. ano.) is a term commonly applied to the lower extremity of the rectum : properly speaking, it is the inferior orifice of the alimentary tube, through which, in the higher orders of animals, the excrementitious portion , of the food, as also the excretions from the di gestive apparatus, are discharged ; for obvious reasons it is endowed with powers to assist in expelling, as also with the faculty of retain ing these for a considerable time : such oppo site but important qualities would infer the existence of a somewhat complicated muscular apparatus, more or less under the influence of the will, as also a structure in other respects worthy of attention.
The presence of an anus indicates a complex system of digestive organs ; hence in many of the inferior or simpler classes of the invertebrate division of animals it is absent, and in many of the superior of this division, as well as in several of the vertebrata, it presents considerable variety as to structure, function, and position. In some of the zoophytes, such as the in fusory animalcules there is no central digestive cavity, and of course no distinct outlet. In them absorption takes place by imbibition through pores into cells, in a manner somewhat similar to a sponge; and most probably excretion (if any occurs) takes place through the same orifices. In others of this class, such as the acalephm, where a rudimental cavity appears in the body of the animal, a single orifice admits the food necessary for its support, and the excremen titious portion (if any) is ejected through the same opening. In the actinim, also, where a distinct stomach exists, and where the retained matter obviously undergoes certain changes, the one orifice serves the two-fold purpose of admission to the food, as well as of exit to its residuum. Even in some of the echino dermata, as the asterim in which the digestive apparatus is more developed, the central cavity becoming more complex, the latter is still but a cul de sac, which can be protruded at the mouth, the only orifice it presents. In other species, however, of this class the anus ap pears ; thus in the English echinus, where the masticating apparatus is so remarkable, this opening exists on the surface of the animal, opposite to the In the sipunculi the anus opens near the mouth, and in the holothurim near the respira tory organ.f In the several families of the articulata, viz. insecta, crustacea, and vermes, the anus exists, and is always found at that end of the animal opposite to the mouth, and most generally on its inferior surface.
In the mollusca it is also present, but it holds situations singularly differing in the different orders and genera of this class ; thus in the cephalopoda, as the cuttle-fish, the rectum opens into a sort of cloaca, which is situated before the neck, and which also re ceives the semen and ova, as well as the secretion from the ink-bag. In the gastero poda, as the slug, it is generally found near the pulmonary cavity. In the patella or limpet, however, it opens on the head, and in the doris on the back, surrounded by a delicate fringe, a sort of branchial tuft. In most of the acephala, except the oyster, the rectum extends along the back of the animal, beneath the hinge, and above the respiratory organs ; it then passes through the heart, and opens above the posterior muscle of the shells, into the cavity of the maulle, or between its edges, the anal opening presenting the appear ance of a fleshy disc or sphincter.