There is, therefore, no systemic heart in Fishes, the aorta itself serving to propel the slow-moving blood in its course through the arterial system.

* For a detailed account of the lymphatic system of Fishes, the reader is referred to the following authors: - Monro,'Anat. and Physiol, of Fishes,' fol.; Hewson, Phil. Trans. 1769; Fohmann, 'Histoirc generate des Lymphatiques des Vertebras,' Heidelberg and Leipzig, fol. 1827.

(1787). The heart (fig. 317, o) is enclosed in a pericardium, and situated beneath the pharyngeal bones and branchial apparatus, the cavity in which it is lodged being separated from the peritoneum by a kind of tendinous diaphragm, and also by a capacious sinus, in which the venous blood derived from all parts of the body is collected preparatory to its admission into the heart.

(1788). The auricle of the heart (fig. 318, b, b) is contained within the pericardium: it varies greatly in form in different fishes, but its capacity is generally considerably greater than that of the ventricle; and its walls are thin, but, nevertheless, present distinct fleshy columns.

(1789). The blood derived from the great sinus before mentioned enters the posterior part of the auricle of the heart by a large orifice, which is guarded by two membranous valves so disposed as to prevent the reflux of the blood during the contraction of the auricular chamber. The ventricle is strong and fleshy; and at its communication with the auricle there is a strong mitral valve. The commencement of the branchial artery (fig. 318, a, d) is so muscular and capacious, that it might almost be considered as forming a second ventricular chamber: this portion, which has been distinguished by the name of the bulb (bulbus arteriosus), is separated from the ventricle by strong valves; and in the cartilaginous fishes, as, for instance, in the Shark, there are several rows of semilunar valves (fig. 318, b, e), so disposed as most efficiently to prevent the blood from being driven back again into the ventricle. In the heart of Lophius (fig. 318, a), the conformation of the cavities is very peculiar: the auricle (b) is large and pyriform, and the ventricle (c) of a globular shape. But the most singular feature in its structure is the valve between the ventricle and the bulb (d): this is a soft fleshy protuberance (e), perforated in the centre, which projects into the cavity of the bulb, and allows the blood to pass freely in one direction; but the sides of the canal collapse, and close the orifice, if the blood is forced back from the bulb towards the ventricle.

(1790). Issuing from the pericardium, the branchial artery runs beneath the centre of the branchial apparatus, dividing into as many trunks as there are branchial arches, to each of which a vessel is given off.

(1791). To each branchial arch are attached a great number of vascular lamellae placed parallel to each other, like the teeth of a comb. The branchial artery, which runs in a groove situated upon the convexity of the corresponding arch, sends off a twig to every one of these laminae; and this vessel, after twice bifurcating, divides into an infinite number of little ramuscules, which run across both surfaces of the branchial fringe, and terminate by becoming converted into capillary veins.

(1792). The radicles of the branchial veins all open into a venous canal which runs along the internal margin of each lamella, and these last terminate in the great vein of the corresponding branchial arch, which runs in the same groove as the artery, but is more deeply situated, and moreover runs in the opposite direction: that is to say, the branchial artery, derived from the heart, and coming from the ventral aspect of the body, diminishes in size as it mounts towards the back, and gives off twigs to the branchial fringe; whereas the branchial vein, on the contrary, receiving blood from the lamellae of the branchia, increases in diameter as it approaches the dorsal region.

(1793). On leaving the gills, the branchial veins assume the appearance and perform the function of arteries. The anterior, even before escaping from the branchial arch, gives off ramifications to different parts of the head; and the heart and parts adjacent likewise receive their supply of arterial blood from a branchial vein.

(1794). The veins derived from all the branchial arches ultimately unite and form the aorta, which evidently corresponds to the aorta of Mammalia, although it has neither auricle nor ventricle at its commencement.

(1795). The aorta, while in the abdomen, runs beneath the spine, and gives arteries to the viscera in the usual manner; but at the commencement of the tail it becomes enclosed in the inferior vertebral arches, by which it is defended to its termination.

(1796). There is yet another set of organs, which, as we ascend from inferior to higher forms of animal life, we encounter for the first time in the class before us - an apparatus for elaborating the urinary secretion, which is peculiar to the Vertebrate classes.

(1797). The kidneys in Fishes are very voluminous: they are situated on each side of the mesial line, immediately beneath the bodies of the vertebra?, and extend along the whole length of the abdomen, not un-frequently reaching to the base of the skull, where their anterior portion (fig. 317, e) lies above the branchial apparatus. The ureters (fig. 317, f) generally terminate in a kind of bladder-like dilatation, the orifice of which is found behind that of the vulva (s).

(1798). Examined minutely, the substance of the kidney is found to be entirely composed of microscopic tubules, which terminate in the ureters: these uriniferous tubes are variously contorted, but of equable diameter throughout; and they end, towards the periphery of the kidney, in blind extremities.

(1799). The skin of these aquatic animals is perpetually lubricated by an abundant mucous secretion furnished by muciparous follicles, or secreted in long tubular organs placed beneath the skin. In the Skate the vessels last mentioned are remarkably large, and their distribution very extensive.