l6 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. 



i. 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,f 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 notrfnore than eight or nine. A Liassic shark, a 

 Cestraciont, Hybodus (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. 



t C'f. Vol. II, of this series. Willey, Amphioxus and Other Ancestors of the 

 Chordates. 



