16 EVOLUTION OF STRUCTURES 
primitive than the most archaic sharks: while, on the 
other hand, to the metameral type of the sharks may the 
structures of the remaining groups of fishes be best referred. 
1. AQUATIC BREATHING 
Respiration in fishes is developed on the primitive chor- 
date plan of ejecting water through gill slits perforating 
the throat wall. The water taken in by the mouth is rich 
in absorbed air, and, as it passes out, is well calculated to 
oxygenate the blood suffusing the sides of the gill slits. 
Among the earliest chordates there seems evidence 
that the gill openings of the gullet were arranged with 
reference to some form of primitive segmentation. Per- 
haps they occurred as well in the region of the mid-diges- 
tive tract, before their location became restricted to the 
gullet. There has been as yet, however, little satisfactory 
evidence * as to the number or conditions of the gill slits 
in very primitive forms. In Amphioxus the gill arrange- 
ment seems clearly a most specialized one: its adult con- 
dition presents an atrium and an elaborate branchial 
basket,t which could hardly have occurred in the lowly 
ancestral chordate. Its early larva, however, is known to 
possess (but in a condition of assymmetry) but a few gill 
slits (seven to nine) from which the many openings of the 
adult branchial basket take their origin, —a developmental 
stage which most closely and most interestingly suggests 
the conditions of higher forms. 
* It has generally been inferred that the immediate ancestors of fishes had 
not many gill slits, probably not more than eight or nine. A Liassic shark, a 
Cestraciont, 7yéodus (p. 85), is known to have had but five; a Permian Pleu- 
racanthid, as in the recent Heptanchus, seven (p. 88); the Lower Carbonifer- 
ous Cladoselache probably seven. 
+ Cf Vol. II, of this series. Willey, Amphioxus and Other Ancestors of the 
Chordates. 
