The cartilaginous arches erected on the hyoid bone do not entirely disappear until the internal gills have ceased to be distinguish able. The circulating system of the decidual branchix consists, in its earliest stage, of a simple artery and vein ; that is, a loop of one vessel. As the larva grows these two vessels become separated by an intermediate system of capillaries. In the latter phase they otfer no remote analogy to the vascular apparatus of the branchix of fishes. The cardiac centres are composed only of a right auricle and one undivided ventricle ; the left or pulmonary auricle remains unevolved until the organic necessities attendant on growth create a ne cessity in the system for the exercise of the pulmonary functions. The left auricle is then superadded, and the chaniber of the ventricle is partially divided by a median partition, and the embryonic organism reaches the maximum limit of development. The pulseless ventral artery', the resultant of the united afferent vessels of the branchix, undergoes oblitera tion through disuse. These general observa tions form no irrelevant introduction to a more special examination of the branchial organs.
Temporary external Gills.— The larval branchice of the frog and toad are less endur ing and less complex than those of the sala mandridte. From the earliest almost to the latest moment of their existence they are furnished with a ciliated epidermis. The gills are not specially ciliated. The whole cutaneous surface in the larva of the frog and toad is similarly endowed. The cilia are in active play for some time before the larva emerges out of the egg : an admirable instance of foresight in the provisions of nature. The covering of the external gills of the ranidx is strictly cutaneous. In this situation, as every where else, the epidermis betrays its real nature by the presence of pigmental cells. It is little less dense than the ordinary covering of the body. .Nor does the vascularity, of these tem porary branchim much exceed that of the rest of the cutaneous surface. These facts pro claim their provisional character. At first they consist of a single minute lobe. This increases into two and then into several. They are cylindrical, not flattened, processes. They bear a single vessel returning upon itself. In this particular of ultimate structure they dif fer from the branchiaz of the salamandridw. In these a capillary net-work is constructed between the artery and vein. This greater elaboration coincides with their longer dura tion. What is ephemeral in purpose is tem porarily formed. This is nature's workman ship. They consist literally of small pro lorwations of 'the skin, which is everyvvhere, as °here, ciliated. At the moment of their fullest development, the larval branchim of the frog consist of four filamentary lobes. These are sessile upon the body or stem of the branchiw ; they are somewhat granular on the surface, and slightly irregular in form. There is also frequently a short additional branch at the base of the posterior one. In these interestinr, organs the movement of the blood is readilydemonstrated. It is a beauti
ful spectacle. It advances in a single current along one side and returns along the other. No sooner have these exquisite organs at tained their greatest development than they begin to diminish in size. They become ob tuse, and are gradually so reduced as to be withdrawn within the branchial cavity, and concealed by a little operculum of the integu ment. The nature of this change of structure, which attends the transition of the branchimi from the external to the internal condition, has never yet been defined by anatomists. It will be immediately described.
The external Gills of the Salamandride ex ceed the former in size, in the number of the appended lobules and in the complexity- of their vascular system (figs. 227, 228.) Like those of the ranidee they are clothed in a vibratile epidermis, numerously starred by pigmental cells, in common with the rest of the body. For some time before the deca dence of these ()roans in the larvee of the triton they cease to exhibit the phenomenon of ciliary vibration. The vibratile epidermis undergoes a change by which the ciliated cell becomes succedeed by the simple. This event foretells the approaching extinction of the parts. In their earliest condition the branchire of the newt discover only four minute simple cylindri cal filaments. Each grows in length and thick ness, and throws out from the inferior surface a double row of pectinated processes. These are more complexly constructed than the pri mitive filaments. They carry not only an afferent and efferent trunk, but an elaborate plexus of capillary vessels. The pigment cells are limited in their distribution to the larger lobes, and to the line of the larger vessels. The epidermis of the secondary processes of the branchim is reduced to extreme tenuity. Through it the eye readily tracks the move ments of the individual blood corpuscles on the bronchial capillaries. These elliptical bodies move like a b oat, their long axes coinciding with that of the channel in which they are travelling, Sometimes several proceed abreast. The diameter of the vessels of the temporary branchim is greater than those of the lungs. In general terms it can be confidently stated that the quantity of blood circulating in the temporary branchim of the amphibia, at the period of their maximum development, is far less in relation to the amount contained in the whole body than that which the lungs, when fully formed, are capable of carrying. This inferior amount of blood is physiologically ex pressive of an inferior functional power in the case of the temporary organs. Their respira tory function is really only supplemental to that of the whole body. The whole cutaneous surface, as in the Nmmatoid annelids, is richly ciliated. It is organised like the bronchia... On these parts, however, the epidermal layer is not so attenuated as that with which the branchim are invested. On these latter there is, however, a very perceptible epidermal co verin„c. Its scales exhibit the ordinary hexa gonal figure.