RENAL ORGANS IN THE LOWER ANIMALS.
Under this head we purpose to refer briefly to such facts in the anatomy of the urinary organs of the lower animals as will serve to render more intelligible the structure and office of the human kidney.
Invertebrata.— That the excretion of urine is a function of great importance is sufficiently manifested by the fact that a special organ for the performance of this office lS found in ani mals very low in the scale of organization.
In insects the urinary glands are usually in the form of long and delicate tubes, but some times present the structure of groups of round vesicles, as in the Carabus, in which the comrnon duct terminates in a small dilatation ; the urinary bladder is likewise present in the water beetles. The excretion is poured into the termination of the intestine, or evacuated con tiguous to the anus.* In the Arachnida, " two long and slender urinary tubes communicate with the beginning of the ccecurn, which seems to stand to them in the relation of an urinary bladder." In the Lamellihranchiata, " the returning veins of the body form a remarkable plexus at the base of the gills near the pericardium, which assumes the form of a distinct glandular organ in the higher Bivalves. The secretion of this venous body abounds with calcareous particles, and the gland was called by Poli the secreting organ of the shell. Modern analysis has detected a large proportion of uric acid in the peritoneal compartment enclosing this venous plexus, and has thus determined it to be the renal organ." t In the Gasteropoda, the urinary gland is a follicular organ attached to the walls of the branchial cavity. In some species of Palu dina the duct dilates to form a small recep tacle. • Among the Cephalopod; the Nautilus pre sents the supposed analogues of the urinary organs in the form of clusters of glandular follicles of a simple pyriform figure, three clusters of such glands contained in mem branous follicles being situated on each of the four branchial veins. The walls of the re ceptacles exhibit in some parts a fibrous tex ture, apparently for the purpose of compressing the follicles and discharging their secretion into the branchial cavity by apertures at the base of the gills. The analogues of these
organs exist in the higher Cephalopoda, in which they are considered to act as kidneys by Mayer ; and Prof. Owen remarks$, " it is more philosophical to conclude that the organs of so important an excretion should be present in all the class, than that they should be represented by the ink-gland and bag, which are peculiar to one order." Vertcbrata. — In Fishes the kidneys are long and narrow ; they are situated on each side of the mesial line, immediately beneath the bodies of the vertebrx, and extending through the whole or the greater part of the dorsal region of the abdomen. They are usually broadest and thickest anteriorly, while they become smaller and approach each other as they extend backwards. Sometimes a single common ureter quits the coalesced hinder ends of the kidneys. In some species the kidneys are thickest at their posterior ends. They have not a well-defined capsule, but their ventral surface is immediately covered by an aponeurotic membrane, against which the peritoneum and the air-bladder, when present, are applied. The renal tissue presents a uniform appearance without di vision into a cortical and medullary portion. The urinary tubules pass immediately into the ureter without the intervention of a pelvic cavity. Malpighian bodi.es exist in the kidneys of the fish as in those of the higher vertebrata ; the structure of these bodies will be fully explained in a subsequent part of this article. The kidneys are supplied throughout their entire length by numerous small branches from the abdominal aorta. In addition to the ar terial blood thus supplied to the kidneys, these organs also receive a large quantity of blood from the veins which proceed from the posterior part of the body. This peculiar system of veins, which was discovered by Bo janus, and more fully described by Dr. Lu dovic Jacobson*, is found in birds and reptiles as well as in fishes. In its primary form it undergoes, according to Jacobson, three de grees of niodification. The first modification exhibits the following form : — From the skin and muscles of the middle part of the body branches arise, which form several trunks, passing separately to the kidneys.