Respiratory Organs of Fishes.
The aquatic type, distinctive almost univer sally of the breathing organs of invertebrate animals, obtains also in the lowest order of the vertebrata. Fishes and the lower amphibia respire on the branchial plan. The difference between a gill and a lung rests more on ap parent than real and ultimate grounds. In the last anatomical analysis this difference vanishes, and the eye is arrested only by the close structural affinities which reduce the two varieties to an essential unity of type. In both, the blood is exposed to the agency of the aerating element by means of reticulated ves sels, furnished with distinct parictes, and pre senting a diameter little in excess of that of the corpuscles' of the blood ; so that these latter must travel through the true respiratory capillary in a single series. This fact denotes the extreme measure to which the subdivision of the blood-stream is carried. It is a funda mental requirement of the breathing organ, that all structures interposed between the blood and the surrounding element should be reduced to the utrnost degree of attenuation. Accordingly, it is found that the epithelium overlying the rete mirabile consists of a single layer of attenuated scales, perfectly destitute of those contained parts which give hulk and density. In no instance whatever within the limits of the vertebma (excepting, as stated already, the branchim of the amphibia) are the true respiratory capillaries covered by a ciliated epithelium. This rule applies also to the branchim even of the higher inverte brata, such as the crustacea and cephalopoda. The gill of the fish differs from that of the crustacean in the extreme minuteness with which the blood current is subdivided, and in the existence of specially parieted vessels ; conditions which denote an ' intensified measure in the function of breathing in the instance of the vertebrate animal. —In the blood of the vertebrata the floating cells are infinitely more numerous, relatively to the bulk of the fluid, than in that of the inverte brata ; — a fact more expressive than the former of the greater activity of the respira tory process in the vertebrate than in the in vertebrate animal.
It is an axiom in physics, that no gas is capable of passing through an organic septum without first assuming the fluid form. This axiom destroys the apparent difference be tween a gill and a lung. In contact with the gill the aerating medium is already fluid: in the case of the lung, it takes this condition only in the act of passing through the partition dividing the blood from the external medium. Between the gill of the fish and the true lung ofthe ver tebrate animal there is discernible, however, this differential character, that in the former the epithelium clothing the active capillary segments forms a thicker layer than in the latter. This is the only true and ultimate anatomical distinction between a gill and a lung.
The preceding general facts will form an appropriate introduction to the study of special details, on which it is proposed now to enter.
The Lancelet (Branchiostoma) occupies the first grade in the vertebrate series. It exhi bits the branchial organs under the least com plex terms. A capacious branchial sac, the dilated cesophagus, occupies the mid-portion of the body. It communicates with the ex terior in front by means of a large (Esophageal opening, and behind by a branchial outlet and a short intestinal canal. The parietes of the stomach display special provisions for breath ing under the character of membranous du plications of the internal surface. These folds are invested with a vibratile epithelium. In this particular the branchim of this fish ap proach those of the lower molluscs, and de part from those of all other fishes. A con venient arrangement of this subject will consist in first studying the mucous membrane of the branchim of fishes ; 2nd. the blood system ; and 3rd. the supporting frame-work.