LARVAL GILLS.—In early life many Fishes acquire larval gills, either as the result of the precocious growth of the normal gills, or by reason of the development of evanescent structures. In the embryos of Elasmobranchs "external gills," in the form of long filiform processes invested by hypoblast, are developed from the walls of all the branchial clefts, including the spiracles, and protrude outwards for some distance through the external apertures of the clefts (Fig. 167, B). They perhaps facilitate respiration within the egg, as they completely disappear after hatching; but there is also reason for believing that they aid in the absorption of nutriment. Similar gills are present in young Holocephali. In some larval Teleosts, as in certain genera of the Osteoglossidae and Mormyridae (e.g. _Heterotis_ and _Gymnarchus_)[306] these structures are remarkably developed (Fig. 239). The young of the Loach (_Misgurnus_) and of the Salmon (_Salmo_) also have the ordinary gill-filaments prolonged externally as filiform structures, which subsequently become reduced to their normal size.[307] In its larval state _Polypterus_[308] has a pair of pinnately-fringed ectodermal or cutaneous gills projecting from the lateral surfaces of the head behind and above the external branchial apertures (Figs. 168 and 281). Apparently as an individual peculiarity the right gill has been retained in a specimen of _P. congicus_ so large as 22 cm. in length, although the left one had entirely disappeared.[309] Each gill is supplied with blood from the ventral aorta by a vessel which ascends the {291}hyoid arch, and is apparently the representative of the artery supplying the hyoidean hemibranch in Elasmobranchs. The efferent vessel of each gill joins the common trunk formed by the union of the efferent vessels of the normal gills of the same side. The cutaneous gills of the Dipnoid _Protopterus_ may also be included in the category of larval breathing organs. They consist of three simple unbranched filaments on each side of the head, and, as in _Polypterus_, are situated at the dorsal extremity of the external gill aperture (Fig. 309). Although usually represented in the relatively young or half grown specimens which, so far, have reached Europe, it is extremely probable that these organs atrophy in older individuals. Similar gills are present in the larval _Lepidosiren_ (Fig. 311), but disappear at a much earlier stage. At no period of its development are larval gills present in _Neoceratodus_.[310]