INTRODUCTORY 235 



which the eyes are contained. Beneath the skull and the 

 anterior part of the vertebral column are a series of transverse 

 bars of cartilage called the branchial arches (Fig. 18 br. 5) be- 

 cause they support the partitions or septa between the gill-clefts. 

 The most anterior of these arches form the jaw cartilages ; 

 these may be considered as one pair of which the lower half 

 forms the lower jaw while the upper half forms the upper jaw. 

 Posteriorly the articulation of the two is attached to the lower 

 end of the first segment of the next arch, called the hyoid 

 (Fig. 18 hy.) while anteriorly the upper jaw is attached to the 

 lower surface of the skull. This mode of suspension of the 

 jaws is highly characteristic of fishes, and occurs in the great 

 majority of the shark-like forms as well as in the bony fishes. 

 In a few cases, however, the upper jaw is firmly united to the 

 skull and the lower jaw is articulated to the upper without the 

 aid of the hyoid arch ; for example, in the lung-fishes (Dipnoi) 

 and Chimseroids. All terrestrial vertebrates in this respect re- 

 semble the lung-fishes. Behind the hyoid arch are five bran- 

 chial arches each segmented into several pieces. The rest of 

 the internal skeleton consists of the cartilages supporting the 

 paired and unpaired fins. Each unpaired fin, excluding the 

 tail fin, is supported by a series of parallel rays, of which the 

 internal segments called the basals are within the surface of the 

 body, while the external parts, called the radials, are in the pro- 

 jecting part of the fin. In the posterior part of the caudal fin 

 the basals cannot be separately distinguished, the radials arti- 

 culating directly with the vertebrae. The terminal part of the 

 vertebral column is bent upwards and the ventral basals and 

 radials are larger than the dorsal, thus the ventral lobe of 

 the tail-fin is the broader and the tail is said to be heterocercal. 

 The pectoral fins are articulated to a transverse bar of 

 cartilage called the pectoral girdle which passes across from 

 one side of the body to the other ventrally ; the part below 

 the articulation is called the coracoid cartilage, the part 

 above it on each side is the scapula. The fin itself has three 

 basals, of which the posterior is the largest, and the external 

 part of the fin-skeleton consists of segmented radials diverg- 

 ing from the base. The pelvic girdle is smaller and there 

 is only one long basal cartilage to which the radials are 

 attached in parallel series. 



