. The Cambridge natural history. Zoology. RESPIRATORY ORGANS 295 - is presented by the Indian Siluroid SaccohrancMis} In this Fish a long caecal diverticulum of the branchial cavity extends backwards on each side from the dorsal region of the first branchial cleft to the tail, and in its course is situated internally to the lateral trunk musculature, and close to the vertebral column (Fig. 172). The walls of the caeca are vascular, but no special respiratory structures are developed within their cavities, which, during life, only contain air. In *S^. singio the right caecum is supplied w
. The Cambridge natural history. Zoology. RESPIRATORY ORGANS 295 - is presented by the Indian Siluroid SaccohrancMis} In this Fish a long caecal diverticulum of the branchial cavity extends backwards on each side from the dorsal region of the first branchial cleft to the tail, and in its course is situated internally to the lateral trunk musculature, and close to the vertebral column (Fig. 172). The walls of the caeca are vascular, but no special respiratory structures are developed within their cavities, which, during life, only contain air. In *S^. singio the right caecum is supplied with blood by an exten- sion backwards of the dorsal portion of the first afferent branchial artery of that side; the left, on the con- trary, being supplied by the corre- sponding portion of the fourth afferent artery of the same side. In S. fossilis ^ both air-sacs are sup- plied by the fourth afferent branchial artery. The efferent vessels join the fourth efferent branchial artery^ right or left as the case may be. With perhaps one or two excep- tions, the accessory respiratory organs of Fishes seem to exist for the pur- pose of enabling their possessors to breathe in air. This is certainly the case with the labyrinthiform organs of Anahas and its. allies, and also in such Fishes as AmpMpnous, Saccohranchus, and the Ophiocephalidae, and probably in others. Nearly all these Fishes are tropical in geographical distribution, more or less amphibious in their habits, and usually possess a remarkable capacity for sustaining life out of water, under conditions which are promptly fatal to ordinary Fishes. Thus, Anahas scandens may be kept alive for days in earthen pots without water, and when free is able to travel short distances on land, especially in the early morning when the dew is on the ground, while Amphipnous frequents. Fia. 172.—Air-sacs of Saccohranchus singio. ^ The air-bladder en- closed in its bony capsule ; , rigbt air-sac ; , left air-sac ; , bulbus aort
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