XIII 



PHYLUM CHORDATA 



215 



from the chondrocranium in the adult ; in the remaining orders, 

 e.g. in the Cod, Haddock, or Perch, they become grafted on to the 

 chondrocranium and so closely united with the cartilage bones that 

 they can be removed only by pulling the whole skull to pieces ; 

 most of the original cartilage frequently disappears in the adult, 

 and the cranium thus be- 

 comes a firm bony mass in 

 which no distinction be- 

 tween cartilage and mem- 

 brane bones is discernible. 



The varying size of the 

 gape, which is so noticeable 

 a feature in the Teleostomi 

 depends upon the inclina- 

 tion of the suspensorium ; 

 in wide-mouthed Fishes 

 (Fig. 828) the axis of the 

 hyomandibular and suspen- 

 sorium is nearly vertical or 

 even inclined backwards : 

 in small-mouthed forms 

 (Fig. 831) it is strongly in- 

 clined forwards and the 

 length of the jaws is pro- 

 portionately reduced. In 

 the branchial arches the 

 pharyngo-branchials of each 

 side are very commonly 

 fused, and constitute what 

 are called the superior 

 pharyngcal bones: the re- 

 duced fifth branchial bars, 

 or inferior pharyngcal bones, 

 bite against them. The 

 Pharyngognathi are dis- 

 tinguished by having the 

 inferior pharyngeal bones 

 united into a single bony 

 mass of characteristic form 

 (Fig. 830, B). The gill- 

 rakers are often very highly 



developed, and may form a mesh capable of retaining even 

 microscopic organisms. 



In the shoulder-girdle, as in the skull, the Chondrostei approach 

 the Elasmobranchs. There is a primary shoulder-girdle con- 

 sisting of large paired cartilages, not united in the middle ventral 

 line, and unossified : each is covered externally by a large scute- 



Fio. 838. Skull of Polypterus, from above. F. 

 frontal ; M. maxilla ; SA. nasal ; Sa. nostril ; Op. 

 opercular ; Orb. orbit P. parietal. The remaining 

 letters point to less important membrane bones. 

 The arrow is passed into the spiracle. (From 

 Wiedersheim's Comparative Anatomy.) 



