290 FISHES CHAP. 
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 Holocephal. In some larval Teleosts, as in certain 
Fic. 168.—Head of young Polypterus. ea.g, Exterual gill of the left side. 
(From Steindachner. ) 
genera of the Osteoglossidae and Mormyridae (e.g. Heterotis and 
Gymnarchus)* these structures are remarkably developed (Fig. 
239). The young of the Loach (JMisgurnus) and of the Salmon 
(Salmo) also have the ordinary gill-filaments prolonged externally 
as filiform structures, which subsequently become reduced to their 
normal size.2_ In its larval state Polypterus? 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.* Each gill is supplied 
with blood from the ventral aorta by a vessel which ascends the 
1 Budgett, Trans. Zool. Soc. xvi. Pt. ii. 1901, p. 126. 
* Gitte, quoted by Balfour, Comp. Embryol. ii. 1881, p. 62. 
3 Steindachner, Sitz. d. k. Akad. d. Wiss. i. 1869, p. 108; Hyrtl, ibid. p. 109; 
3udgett, op. cit. p. 118. 
* Boulenger, P.Z.S. 1899, p. 554, 
