INTERNAL ORGANS 165 



backwards in one vertebra and meet a similar process projecting 

 forwards in the next. 



It is a general rule that the more deep-seated organs are far 

 less liable to modification than the more superficial structures, 

 such as the scales, fins, and so on. Thus, although they show a 

 number of diflferences in the various orders, suborders, and 

 families, often of considerable value in classification, the skulls 

 of Bony Fishes, apart from the jaws, do not present many very 

 striking modifications. This is true to some extent of the 

 vertebral column, but here there are one or two remarkable 

 adaptations worthy of consideration. In the deep-sea Chauliodus, 

 for example, which is in the habit of throwing the head back 

 when striking at its prey in order to bring the huge lower canine 

 teeth into play {cf. p. 134), the first vertebra immediately 

 behind the skull is enormously enlarged, being several times 

 larger than any of those following. This serves to take the 

 strain when the head is suddenly jerked back, and at the 

 same time provides additional surface for the attachment of 

 the muscles moving the head (Fig. 67D). In some of the 

 members of the genus Eustomias, oceanic fishes of the suborder 

 of Wide-mouths (Stomiatoidea) , the anterior part of the vertebral 

 column is unique in being incompletely ossified and with the 

 notochord bent to form one or two distinct loops. The first 

 vertebra is normal, but this is followed by six or seven without 

 centra and made up of isolated bony elements. This curious 

 modification is undoubtedly related to the violent movements 

 of the head involved in protruding the jaws and in wrestling 

 with large prey, the incompletely ossified and bent anterior 

 portion giving flexibility and acting as a kind of shock absorber 

 (Fig. 67c). It has been suggested that the opening out and 

 closing up of the bends, and the corresponding movements of 

 the jaws, may assist in swallowing prey. In Stylophorus, another 

 oceanic form with protractile jaws, whose feeding habits have 

 been already described {cf. p. 118), a similar strain due to the 

 backward jerk of the head is provided for by a complicated 

 system of interlocking among the first few vertebrae by means 

 of special bony processes (Fig. 67B). Finally, in the Sword- 

 fishes {Xiphiidae) and Spear-fishes (Istiophoridae) , whose violent 

 charges on whales and other Cetaceans have been considered 

 in an earlier chapter {cf. p.i 14 ), the vertebrae have undergone 

 some marked modifications clearly designed to give power 

 to resist the shock of such encounters, the interlocking processes 

 being very powerful (Fig. 67A). The changes undergone by 



