The larger animals which inhabit the mighty river and the network of streams and pools which surround it on both sides, have been described; but numerous smaller creatures dwell within it, equally curious, and many totally unlike those to be found in other parts of the world. It has generally been supposed that, of all creatures, fish are the most destitute of parental feelings, and that from the moment the eggs have been deposited in the sand or mud, they are allowed to struggle into existence as best they can, to do battle with their foes, and the numerous dangers to which they are exposed. In the acara, however, we have an example of parental care and watchfulness unrivalled by any terrestrial animal. The male of this curious fish has a conspicuous protuberance on the forehead, wholly awanting in the female and the young. Somehow or other, the eggs of the female are conveyed into the mouth of the male, the bottom of which is lined by them, between the inner appendages of the branchial arches, and especially into a pouch formed by the upper pharyngeals, which they completely fill. They are there hatched; and the little ones, freed from the egg, are developed until they are in a condition to provide for their own existence. In their head there is a special lobe of the brain, similar to those of the triglas, which sends large nerves to that part of the gills protecting the young, thus connecting the care of the offspring with the organ of intelligence. In this curious cavity of the father's head the young fish are found in all stages of development,--the more advanced, a quarter of an inch long, and able to swim about, full of life and activity. These appear to exist outside the gills, within the cavity formed by the gill-coverts and the wide branchiostigal membrane. The eggs remain in the back part of the gills. The parent's care does not appear to cease even when the young are fully developed, but he allows them to swim in and out, and try their powers, if not to search for food; and when danger appears, opens his mouth, when they all swim back again in a shoal, for safety. The natives assert that some species, at all events, are not actually developed in the parent's head, but are laid and hatched in the sand, the male and female watching carefully over them; and that the father only takes charge of them when they are hatched, and receives them within his mouth to protect them from danger. From the observations of Professor Agassiz, however, there is no doubt that in some species, at least, the whole process of development is begun and completed in the gill cavity. The species which lay their eggs in the sand belong to the genera Hydrogonus and Choetobranchus. They build a kind of flat nest in the sand or mud, in which they deposit their eggs, hovering over them until the young are hatched. Curious also is the little bill-fish--the lymnobellus--with its long beak.