THE FISH SKULL AS A NATURAL MECHANISM 



The characteristics of the skulls of fishes are mentioned in thousands of taxonomic 

 papers solely because they are valuable as marks for distinguishing one kind of fish from 

 another. A few modern monographs, chiefly by Allis, deal very thoroughly with several 

 types of teleost skulls in the interest of descriptive morphology. Numerous papers or 

 books, by Smith Woodward, Stensio, Watson and others, give the palseontological data 

 concerning the evolution of particular skull types, while still others, including some by 

 Ridewood, Starks, Boulenger and Tate Regan, record the detailed cranial osteology either 

 of particular groups or of special parts of the skull. Up to the present time, however, but 

 few attempts have been made to study the fish skull as a "natural mechanism" in order to 

 show the functional connections of its various parts. 



A fish skull may fairly be called a natural mechanism because it is a product of nature 

 and because it has the properties of mechanisms in general plus certain properties not found 

 in human mechanisms. It is also part of a complete natural machine, the fish itself. 



This machine captures stored-up solar energy from the environment and later utilizes 

 part of this energy to operate its mechanism for the capture of more energy. It also nor- 

 mally steers away from danger and into favorable locations and uses another part of the 

 energy in preparing the raw material of the next generation. 



Form and Evolution of the Branchiocranium 



The branchiocranium, including the jaws and branchial arches, obviously plays a vital 

 part in the capture and turn-over of solar energy. It comprises the so-called visceral 

 arches and all their osseous and cartilaginous appendages. Seen from below (Fig. 5), 

 these arches form a series of inverted V's with the apices pointing forward. The successive 

 apices are connected by median basal pieces. The first V is the mandible, those behind it 

 are the lower segments of the hyoid and branchial arches. Seen from the medial aspect 

 (Fig. 1), the branchial arches form a series of V's with the apices directed backward. The 

 origin of this arrangement is discussed above (p. 83). 



It is well known that there are two sorts of jaws, outer and inner. The outer upper 

 jaws are bony dentigerous tracts, the premaxillae and maxillae, which in the oldest ganoids 

 form part of the bony facial mask. They were at first fastened tightly to the inner or 

 primary upper jaw (the palato-pterygo-quadrate arch). In the later ganoids and teleosts 

 the premaxillae and maxillae become movably pivoted pn the ethmo-vomer block. After 

 this step was achieved these elements enjoyed a wide adaptive radiation in the teleosts 

 (Fig. 283). 



The Inner or primary upper jaw seen from below forms an inverted V with the apex 

 pointing forward. Its cartilaginous core, which in the shark is represented by the palato- 

 pterygo-quadrate, gives rise in the teleosts to the paired quadrate, metapterygoid, mesoptery- 

 goid, "pterygoid" (ectopterygoid) and the palatine bones, the latter bearing dentigerous 

 plates. 



The mandible consists of an outer dermal shell, the dentary bone; and of an inner core, 



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