& 



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 Holocephali. In some larval Teleosts, as in certain 



Fig. 168. — Head of young Polypterus. ex.g, Exterual gill ot 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 (Misgtirnus) and of the Salmon 

 {Salmo) also have the ordinary gill-filaments prolonged externally 

 as filiform structures, which subsequently become reduced to their 

 normal size.^ 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 



' Budgett, Trans. Zool. Soc. xvi. Ft. ii. 1901, p. 126. 

 ^ Gotte, quoted by Balfour, Comp. Enibryol. ii. 1881, p. 62. 

 ' Steindachner, Sitz. d. k. Akad. d. Wiss. i. 1869, p. 103 ; Hyrtl, ibid. p. 109 ; 

 Budgett, op. cit. p. 118. 



^ Boulenger, P.Z.S. 1899, p. 554. 



