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. 11 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.t+ There are thus the gill-encasing derm frills of 
the archaic sharks, Cladoselache, Chlamydoselache, and 
Acanthodes (pp. 78-83), or of Chimzeroids (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, ¢,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). 
+ This condition appears to have been possessed by the Lower Carbo- 
niferous Cladoselache. — 
