likewise, and the short cut completes the circuit. In order to do this they have, in the Anura, to form new connections with the trunks of the afferent vessels. The arterial arches themselves are modified as follows:–The first pair become the carotids, the second form the right and left aortic arches, while the third and fourth unite and are transformed into the pulmonary arteries and "ductus Botalli," the last arterial arch having previously sent a branch into the developing lungs. In the Anura the third arch obliterates. The gills and clefts present various modifications. The Urodela possess three pairs of gills, one each upon the dorsal {42}half of the three branchial arches, just near the upper corners of the clefts; and the skin of the body is continued upon the stem of each gill, pigmented like the rest of the surface of the body. Such a gill is more or less like a blade, standing vertically, and is composed of a stem of connective tissue, thick at the base, and, as a rule, carrying two series of fine lamellae, which, however, do not form two opposite series, but hang downwards, being, so to speak, folded down, so that the upper surface of the stem is bare, and carries the lamellae on its under side. In the Axolotl some of these lamellae are further subdivided. In _Necturus_ they are enormously increased in numbers, but are rather short, and they stand no longer in two rows, but are crowded into one. Those of _Proteus_ form two rows of dendritic filaments; those of _Siren_ are likewise much ramified. The larvae of the Urodela have four clefts. In the adult _Siren_ these are reduced to three, the first, namely, that between the hyoid and the first branchial arch, being closed up. In _Necturus_, _Proteus_, and _Typhlomolge_ the clefts are further reduced to two, owing to the closing up of the first and last, only those between the first, second, and third arches remaining. _Amphiuma_, and usually _Cryptobranchus alleghaniensis_, possess only one pair of clefts, while in _C. japonicus_ and in the Salamandridae all the clefts are abolished. The gills of the Urodela are always uncovered, although a short operculum is formed from the posterior margin of the hyoidean arch; the halves of this fold meet below the throat, and persist in various terrestrial and aquatic species as the "gular fold." It reaches its greatest size just before metamorphosis, but scarcely ever produces a proper outer gill-chamber, and it does not cover the gills owing to their rather pronounced dorsal position. It is perhaps best developed in _Typhlomolge_, and even there its dorsal portion is continued upon the first of the three broad vertical and short-fringed blades which form the gills. A description of the gills of the Apoda will be found in the systematic part.