20 GILL CHARACTERS 



the throat muscles draw in or eject the water used in 

 respiration. On the gullet wall, the gill bars, now seen 

 to be closely drawn together, have acquired marginal 

 outgrowths, or gill rakers, GR, which form an inter- 

 locking screen across the gill openings and prevent the 

 escape of food organisms. So perfect may this apparatus 

 become that the opening and closing gill bars may retain 

 even microscopic life.* 



Between the conditions of Figs, n and 12 there occur 

 many transitional forms. 



To protect the gill region, specialized devices are known 

 to have been evolved early in the history of fishes, 

 the more early if, as Garman has supposed, the gill fila- 

 ments in primitive sharks protruded at the sides of the 

 head.f There are thus the gill-encasing derm frills of 

 the archaic sharks, Cladoselache, Chlamydoselache, and 

 Acanthodes (pp. 78-83), or of Chimaeroids (p. 100). These 

 protective structures, the writer believes, may well have 

 originated independently even within the limits of sub- 

 groups. They have certainly no direct relation to the 

 opercle of bony fishes. 



Modes of respiration by-gill filaments have been found 

 in endless variety among fishes, clearly dependent in the 

 majority of cases upon environment. Thus fishes that 

 require a temporary existence out of water will be found 

 to have specialized spongy gill filaments and a closely fit- 

 ting gill cover to keep moistened the respiratory organs 

 (e.g. Callichthys, p. 172). 



* Thus in many bony fishes, e.g. mullet or Brevoortia (menhaden), the 

 inner margins of the gill bars are fringed with what appears like the finest 

 gauze, each gill raker giving off primary, secondary, and tertiary branches. A 

 somewhat similar condition occurs in the shark, Selache (p. 90). 



t This condition appears to have been possessed by the Lower Carbo- 

 niferous Cladoselache. 



