GREGORY: FISH SKULLS 85 



III. Selection for larger prey and more active swift-swimming led to: 



(1) Need for improved respiration: hence 



(a) Great enlargement and folding of gill filaments (compare cyclostomes with 



sharks); 



(b) Strengthening of zonal and oblique muscles around gill region; 



(c) Bending of irregular gill-basket into gill-arches; 



(d) Formation of joints in gill-arches in adjustment to muscle pulls; 



(2) Enlargement of oromandibular arch and its muscles with change of function from 



respiration to ingestion of struggling prey; 



(3) Sharp flexure of oral and branchial arches backward, so that they overlap each other. 



IV. Continuance of (3) above, resulting in opercular flaps. 



In the development of modern fish probably much of this evolutionary history has 

 been greatly foreshortened, so that the elements of the visceral arches are emphasized in 

 very early stages. 



Synopsis of Ten Stages Leading to the Typical Percomorph Skull 

 The structural stages of evolution, several of which almost certainly do not lie in a 

 direct phyletic sequence, are as follows: 



1. Undiscovered and hypothetical prechordate stage, possibly resembling in funda- 

 mental characters the " Tornaria" larva of Balanoglossus. Pre-Cambrian. 



2. Undiscovered early protochordate stage, presumably like the eaj-ly stage of Amphi- 

 oxus in basic features; namely, with bilateral symmetry and a sequence of enterocoelic 

 pouches giving rise to contractile locomotor sacs; head incipient. Cambrian. 



3. Primitive ostracoderm (Fig. 2), with typically chordate bilateral and cephalo- 

 caudal differentiation; head comprising capsules for olfactory, optic and static organs in 

 antero-posterior sequence; "agnathous," i.e., jaw cartilages, if present, not different in 

 character from the other branchial arches. Ordovician. 



4. Primitive undiscovered osteichthyan gnathostome with orobranchial arches shark- 

 like but covered with ganoid plates and provided with true teeth (common ancestral stock 

 of Crossopterygii and Actinopterygii). Silurian. 



5. Primitive palaeoniscoid actinopterygian (e.g., Cheirolepis) (Fig. \2A); predaceous 

 skull with large backwardly-inclined maxillary and suspensorium; eyes far forward; "asym- 

 metric" arrangement of cheek plates; median and paired gular plates; parasphenoid not 

 extending behind infundibulum. Devonian. 



6. Primitive protospondyl (Jcentrophorus) (Fig. 21), with relatively small mouth; eyes 

 dominant with "concentric" arrangement of circumorbital and postorbital bones, essentially 

 as in larval teleosts of the present time. Permian. 



7. Primitive amioid (Eugnathus) (Fig. 27), with larger predaceous mouth; ganoine 

 surface retained; median gular plate present; mandible retaining "splenial" (coronoid) 

 elements. Triassic. 



8. Primitive teleostean (Leptolepis) (Fig. 30); ganoine surface reduced; "postorbitals" 

 absent; median gular plate reduced or absent; maxilla forming greater part of oral border; 

 two supramaxillaries. Jurassic. 



9. Progressive deep-bodied clupeoid (Ctenothrissa) (Fig. 42); transitional between num- 

 bers 8 and 10; two supramaxillaries. Cretaceous. 



