GREGORY: FISH SKULLS 



83 



According to the hypothesis here put forward, the primitive chordates were compara- 

 tively sluggish, bottom-living forms with a large oropharynx, feeding on small organisms 

 by ciliary ingestion. Progressive selection for larger prey and .for free-swimming, preda- 

 ceous habits led to the enlargement of the oromandibular and branchial arches and of their 



■mandibular 

 nerire 

 eyemuscies i ^' 



i V 



supe/Ziciai constr/dor 

 muscles of^iUarches 



lateral 

 canais 



Meckels 

 cartilage 



adductor musc/es 

 ofjaurs 



Fig. 6. Head of CUamydoselachus anguineus. Redrawn and slightly simplified by Mrs. Helen Ziska after the color plate 

 in Allis, 1923, PI. IV. 



constrictor, adductor and interbranchial muscles. The rhythmic contraction of these 

 muscle bands in connection with both respiration and deglutition would tend to bend and 

 fold the enlarging orobranchial arches and to break them up into joints. In this connection 

 Allis has long recognized that the subdivision of the mandibular and branchial arches was 

 conditioned by the development of biting jaws (1925a, p. 75). SewertzoflF (1927, p. 520), 

 in comparing the embryo shark with the larval cyclostome, notes that the flexure of the 

 orobranchial arches has been conditioned by the activity of these muscles. Goodrich 

 (1930, p. 441) also remarks that "The segmentation of the arches is perhaps secondary; 

 it is probably related to the development of special branchial muscles and allows the walls 

 of the pharynx to be expanded and contracted for breathing and eating purposes." Fur- 

 ther enlargement beneath the zone of constrictor muscles, together with the advantages of 

 a narrower head in rapid swimming, would lead directly to the obliquity of the jaws and 

 arches as seen from below (Fig. 5) and to their flexures in the lateral view. The enlarge- 

 ment and folding up of all these arches is evidenced by the sharp turning of the cranial 

 nerves which supply the constrictor muscle bands, as shown (Fig. 6) in the dissections of 

 Chlamydoselachus figured by Allis (1923a). 



