The pancreas is flattened and lobed (? absent in Siren and Proteus).
The heart lies anteriorly, as in Fishes, except in Gyninophiona. It consists of a sinus venosus divided into a larger right moiety, receiving the venae cavae, and a smaller left moiety receiving the pulmonary veins: of two auricles separated by a septum, which is complete in Anura but often much fenestrated in Urodela, e. g. Proteus, Caecilia, Triton; and of a single ventricle. There is always a contractile conus arteriosus, as in Dipnoi and Elasmobranchii, the muscles of which are striated. It is separated usually by a row of three valves from the ventricle and of four from the truncus aortae. One of the anterior valves is of great length and takes a spiral course in the conus corresponding to the typical spiral twist of that structure. This twist is sometimes lost. The number of valves may be increased. In Caecilia (Gymnophiond) the conus is very short, and there is but one row of valves, probably the distal. The truncus aortae is short, especially in the Anura. A longitudinal septum crosses it from side to side in such a way that it cuts off a ventral cavity, from which the first or carotid arch, and the second or aortic arise, and a dorsal, from which the third and fourth arches arise. These vessels are all closely united at their origins.
In the Tadpoles of both Urodela and Anura there are four vascular arches. There is a direct anastomosis between the three first branchial arteries and veins in Urodela, but a connection through the branchial capillaries only in Anura. The fourth arch never supplies a gill, but gives off the pulmonary artery. The first arch gives off the internal and external carotids before it unites with the remaining three. At the metamorphosis the first arch either retains its connection with the second as in Triton and Derotremata, or loses it, as in Salamandra and Anura. The second enlarges and becomes the aorta: the third dwindles, and is either lost in Triton, Derotremata and Anura, or persists irregularly in Salamanders. The fourth arch enlarges and supplies the lungs: and in Anura a large pulmo-cutaneous vessel arises close to the place where the third arch was connected with it. The Perennibranchiata retain the condition necessary to branchial respiration as seen in Urodela, but Menobranchus and Proteus have no trace of the fourth arch except its anastomosis with the third, which serves as the origin of the pulmonary artery, and Siren has but a small rudiment of it springing from the third arch.
There is a carotid gland, so-called in Anura, Salamandrina, and in Amphiuma alone among Derotremata. It is absent in Perennibranchiata and Gymnophiona. It is produced, as in the Frog, by the development of anastomoses between the external carotid, i.e. lingual artery, and the first branchial artery. The aortae unite below the backbone. In Anura the left arch gives off the caeliaco-mesenteric artery before uniting with the right. Two superior venae cavae, an inferior cava, epigastric vein and renal-portal system are always present, as in the Frog (see pp. 76-7). There are subcutaneous lymph-spaces, of great size and distinctness in Anura; lymph-vessels in the skin, etc.; a lymph sinus between the two lamellae of the mesentery, which is connected with the lymph vessels and spaces of the stomach and viscera, and with a large subvertebral lymph space (cisterna lymphatica magna). The Anura have an anterior and posterior pair of lymph hearts; the Urodela, like the Rep-tilia, only a posterior pair1. They connect the lymph system with the vascular.
All Amphibia, with the exception of some Gymnophiona (?), pass through a branchiate condition. The Tadpole possesses three pairs of external ciliated gills attached near the dorsal ends of the three first branchial arches. The most anterior of these gills is the largest: their form varies. In the perennibranch Urodela they are retained, and are either lobed (Menobranchus), dendriform (Proteus, Siren), or fringed. In the larvae of other Amphibia which lose the gills, they are more or less lobed, or dendriform, as a rule. In the Anuran Notodelphys they are bladder-like, as in Caecilia compressicauda among Gymnophiona. Epicrinm glutinosum in the same order has three pairs of dendriform gills2. There are four branchial slits, reduced to three in Siren, or to two between the first, second and third branchial arches in Proteus and Menobranchus. The adjacent edges of the arches in Urodela are furnished with interlocking tooth-like processes, as in Fish, which become very much complicated in the Anuran Tadpole. The external gills are soon lost in Anura and replaced by internal foliate gills. The gill-slits are also covered, as in the larvae of Salamanders, by a membranous opercular fold, which reduces the external apertures. They are eventually closed.
In one group of Ichthy-oidea, the Derotremata, the slit between the third and fourth branchial arches is retained permanently after the shedding of the branchiae. This event does not always take place at the same date in the life of the larva.
The inlet from the pharynx to the lungs is guarded by two arytenoid cartilages in Urodela, to which are added a circular cricoid cartilage in Anura. There are both constrictor and dilator muscles acting on these cartilages. There is a trachea of considerable length in Menopoma, Am-phiuma and Gymnophiona,a.nd bronchi in Pipa and Dactylethra: the cartilage supports of these tubes do not form rings. In other Amphibia the air passages are only rudimentarily represented. There are two lungs, ellipsoidal sacs in Anura, more or less cylindrical in Salamanders. As to Perenni-branchiata they are short in Menopoma, long in the others, and much contracted for part of their length in Proteus. The left lung is rudimentary in the snake-like Gymnophiona. The inner surface of the lungs is areolated, but in Menobranchus and Proteus it is smooth. A peritoneal fold suspends each lung in the two last-named Amphibians and in Menopoma to the dorsal wall of the coelome.
1 Many contractile lymph-hearts have lately been discovered in Salamandra maculosa and Siredon (Axolotl); Weliky," Z. A. vii. 1884. So too in the Frog's tadpole, Id. Z. A. ix, 1886.
2 External gills are not known to exist in other Gymnophiona. The young Epicrium glutinosum, after losing its gills on hatching, has two gill slits. Caecilia oxyura when young has a single pair of slits; C. recurvirostra has none.