This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1968). The os hyoides of the tadpole, at an early period of its development, supports four branchial arches (fig. 346, a, l, 2, 3, 4), which bound three branchial fissures, through which, as in a fish, the water escapes from the mouth. The branchial arches 2 and 3 are studded on each side with cartilaginous points, and the arches 1 and 4 have similar points on one side only; so that when the arches are approximated, as they can be by an elaborate temporary set of muscles provided for the purpose, the cartilaginous teeth lock into each other so accurately that the branchial fissures are completely and firmly closed - a provision which is evidently indispensable in order to allow the tadpole to fill its lungs with air.
(1969). The above is the condition of the branchial portion of the hyoid apparatus before the metamorphosis of the tadpole has made much progress; and from this time a series of changes begins of a most curious and interesting description.
(1970). When the metamorphosis has commenced, the os hyoides and branchial arches assume the appearance represented at fig. 346, b. The pieces 8 and 9 are no longer both cartilaginous, the latter having become entirely ossified. The branchial arch 1 is likewise converted into bone; and its upper surface, being considerably enlarged, is now connected with both the pieces marked 10 and 11. The three cartilaginous pieces 5, 6, 7, in fig. 346, a, are consolidated into one, while the branchial arches 2, 3, 4 become much reduced in size, the branchise approach each other, and the cartilaginous points with which they are provided adhere together, so that from hour to hour, so to speak, the mass (2, 3, 4) composed of the three united branchial arches becomes insensibly obliterated, and in a very few days is entirely absorbed, While this absorption is going on, the branchial arch (1) assumes greater consistency, its inferior extremity becomes directed outwards, and it loses the little cartilaginous teeth previously appended to it; the os hyoides thus assumes the simple form represented in fig. 346, c. Lastly, the cartilage 6 disappears, and the complex branchial apparatus of the tadpole becomes converted into the permanent and comparatively simple os hyoides of the Salamander, depicted in fig. 346, n.
* Recherches anatomiques et physiologiques sur les organes transitoires et la metamorphose des Batraciens, par J. Gr. Martin St.-Ange (Annales des Sciences Naturelles, xxiv.).
Fig. 346. Metamorphoses of the os hyoides in the Tadpole. (After Dr. St.-Ange).
(1971). The branchial arches 2,3,4, Dr.St.-Ange remarks, are absorbed in proportion as the circulation becomes modified, their atrophy depending upon the change which takes place in the course of the blood, owing to the dilatation of the anastomotic vessels (fig. 345, e e e) and the enlargement of the pulmonary arteries (6.) It is, therefore, owing to a kind of revulsion produced by the afflux of the blood towards the pulmonary organ instead of towards the branchiae, that the atrophy of the branchial capillaries, and subsequently of the whole branchial apparatus, is produced.
Fig. 347. Course of the circulation in Lepido-siren. (After Owen).
(1972). We must in the last place, before leaving the consideration of the circulating system of the Reptilia, describe that of the Lepido-siren, a creature so exactly intermediate between the two classes, that it is really difficult to determine whether it ought most properly to be called a fish provided with lungs, or a reptile with the circulatory organs of a fish.
(1973). The heart resembles that of a fish, and consists of a single auricle (fig.347, a), a ventricle (6), and bulbus arteriosus (c.) The vena cava (e), bringing the vitiated blood from the system, terminates at once in the auricle, which is represented in the figure as laid open; but the pulmonary vein (f), whereby the aerated blood is brought from the lungs (m m), passes along as far as the auriculo-ventricular opening, where it empties its contents into the ventricle by a distinct orifice, protected by a cartilaginous valvular tubercle.
(1974). It is, therefore, only necessary in this case to dilate the pulmonary vein previous to its termination, to make a heart with two auricles; but, as Professor Owen observes, the same advantage is secured to the Lepidosiren in a different manner; for, while it still retains the dioecious type of the heart of the fish, the continuation of the pulmonary vein prevents the admixture of the respired with the venous blood until both have arrived in the ventricle.
(1975). The aorta, or, rather, the bulbus arteriosus (y), in this interesting creature, fulfils at once the office of a systemic, a branchial, and a pulmonary artery. It gives off on each side six vessels, which correspond to the six cartilaginous branchial arches; of these arches four, namely the 1st, 4th, 5th, and 6th, support gills, so that the arteries belonging to them (1, 4, 5, 6) are, as in fishes, distributed over the branchial fringes, and are thus true or functional branchial vessels. But the 2nd and 3rd arches have no gills appended to them; so that the arteries (2, 3,) belonging to these arches do not divide, but are continued round to the dorsal region, where they unite to form an aorta, as in Menopoma (fig. 343); moreover, before their union to form the systemic trunk, they give off the pulmonary arteries (I, m) by which the pulmonary circulation is supplied. Thus each contraction of the ventricle of the heart drives the mixed blood derived from the venae cavae and pulmonary veins, first, to the gills; secondly, to the aorta, through the vascular trunks (2, 3); and thirdly, to the lungs, through the pulmonary artery (I, m); so that from this arrangement, whether the creature be placed in water or in air, respiration is carried on efficaciously either by the pulmonary or branchial apparatus vicariously.