GREGORY: FISH SKULLS 431 



and producing a high apex of the back. An extreme case of this tendency is shown in Fig. 

 297. At the same time the occiput deepens vertically but not as fast as the back, so that the 

 skull roof slopes upward to the apex. In long-bodied teleosts, on the other hand, rapid 

 multiplication of segments takes place in a fore and aft plane, while the occiput remains low 

 (Fig. 297C). In either case forward growth of the snout naturally tends to lower the slope 

 of the skull roof (Figs. 291 £, 290C). In still other teleosts the transverse growth of the 

 mouth (Fig. 293C) results in a wide, low head with a broad neurocranium. 



We do not know exactly what conditions the dominance of one or another of these three 

 more or less opposing tendencies toward length, height or width respectively, which have 

 profound effects on all the bones of the neurocranium and branchiocranium (Fig. 294); but 

 by analogy with mammals we may infer that these long, high or broad types have been 

 produced through gradual changes in the partly hereditary qualities of the enzymes pro- 

 duced by the endocrine glands. 



In most fishes, the food of the larvae differs widely in size from that of the adult, as do 

 the jaws and neurocranium. In small-mouthed fishes there may have been finally a 

 tendency to retain the relatively small larval mouths in the adult (see p. 416 above). 



The Form and Evolution of the Neurocranium 



From the combined evidence of palaeontology and taxonomic ichthyology there can 

 be no substantial doubt that the several chief classes of true fishes (including the sharks and 

 their allies, the crossopts and the actinopts) arose from animalivorous types. That is, all 

 typical fishes are and were predaceous forms equipped to pursue and devour living prey. 

 Some of the more specialized forms seek sedentary or slowly moving food and must therefore 

 be equipped, for example, to break down the defensive armor of bivalves or crustaceans. 

 Others by enlarging their mouths and developing gill-strainers browse on the limitless 

 pastures of copepods and minute shrimps. But all such specialized forms have more 

 conservative relatives that still pursue medium-sized prey in the way followed by their 

 remote ancestors. 



Consequently the neurocranium in any primitive type is adapted to support the organs 

 of a fish that pursues animal prey in a fluid medium. It resists the various forces that 

 impinge upon it in such a way as to afford both an immobile support and a protective cover 

 for the cephalic sense organs, the brain and related parts. It also affords a fulcrum or 

 anchor for the branchiocranium as well as for the backbone. 



The neurocranium is a sort of passive cast or deposit in osseus tissue of spaces left 

 between the more active parts of the head. Like the branchiocranium, it is a product of 

 the growth of the vascular and connective tissue systems. Some of the factors affecting its 

 form and evolution are considered below. 



Mutual Adjustment of Head and Body 



Stream-lining. — A modern student of piscine morphology, especially if he has studied 

 fishes in their own habitat and has seen how adroitly they snatch and dispose of their chosen 

 type of food, must admit that the various parts of any fish are in fact correlated with each 

 other in such a way as to enable the fish to execute complicated manoeuvers with the greatest 

 ease. This easy progress of the fish through the fluid medium is secured by the stream-lining 

 of its body, that is, the body-form is presumably such that the fish at its highest normal 

 speed will leave a flowing rather than a turbulent wake behind it. 



