In the Siluroid genera _Clarias_ and _Heterobranchus_ the accessory organ takes the form of branched, arborescent and highly vascular structures, developed as outgrowths from the dorsal extremities of one or two branchial arches, and enclosed within a posterior and dorsal expansion of the proper branchial cavity (Fig. 171). {294}Another example of these interesting structures occurs in _Chanos salmoneus_ and a few other Clupeidae[320] in the shape of a coiled gill-like organ ("gill-helix"), which is supported by the dorsal segment of the fourth branchial arch, and enclosed in a similarly curved caecal extension of the branchial cavity. Each gill derives its blood from the fourth afferent branchial artery, the corresponding efferent vessel joining the fourth efferent branchial artery. A similar spirally-coiled "gill-helix" is found also in _Heterotis ehrenbergii_,[321] amongst the Osteoglossidae, and in several species of Characinidae.[322] [Illustration: FIG. 171.—Accessory respiratory organ of _Clarias_, as seen after the removal of the left operculum. _a_, Anterior arborescent organ; _b.a^{1-4}_, the first four branchial arches and their holobranchs; _d.b.c_, dorsal extension of the left branchial cavity; _f_, modified gill-filaments; _op_, base of the operculum; _p_, posterior arborescent organ.] In other Teleosts the accessory breathing organ assumes the condition of paired lung-like outgrowths of the branchial cavity. Thus, in one of the Symbranchidae, the Indian "Cuchia Eel" (_Amphipnous cuchia_),[323] there is a pair of small bladder-like sacs, with membranous and vascular walls, each of which opens into the branchial cavity above the first gill-cleft, and is supplied with blood by the afferent branchial artery of the gill-less first branchial arch. An extreme modification in the same direction {295}is presented by the Indian Siluroid _Saccobranchus_.[324] 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 extension backwards of the dorsal portion of the first afferent branchial artery of that side; the left, on the contrary, being supplied by the corresponding portion of the fourth afferent artery of the same side. In _S. fossilis_[325] both air-sacs are supplied by the fourth afferent branchial artery. The efferent vessels join the fourth efferent branchial artery, right or left as the case may be.