IX SKELETON 319 



— consists of an outgrowth of cartilage at the dorsal end of Meckel's 

 cartilage which spreads forward immediately over the mouth, supporting 

 the upper set of teeth, and becoming segmented off so that its connexion 

 with Meckel's cartilage forms a freely movable joint. There is thus 

 brought about the hinged jaw-apparatus, enabling the mouth to be 

 opened and closed, that is one of the characteristic features of the typical 

 vertebrate. 



The skeleton of the second or hyoid visceral arch — lying between the 

 spiracle and the first branchial cleft — becomes modified to a much less 

 extent. The main feature to notice is that its upper portion (hyo- 

 mandibular) becomes considerably enlarged in correlation with the fact 

 that it plays an important part in supporting the lower jaw which is 

 bound to it by strong ligaments. Here we have a feature that is 

 characteristic of several types of fish, and such a type of skull or head- 

 skeleton — in which the hyoid arch takes part in the suspension of the 

 jaws from the cranium — is termed hyostyllc. 



It will be noticed in Fig. 133 that the gill-rays attached to the hyoid 

 arch are particularly large and strong and that most of them are branched. 

 This is correlated with the fact that the visceral arch in question (II) is 

 comparatively bulky, not being reduced to a thin septum as is the case 

 with the arches further back in the series. 



The two small cartilages " I " in Fig. 133 are termed labial cartilages : 

 they are embedded in the edges of the mouth and are probably of no 

 particular morphological significance. 



The thin marginal parts of the fins are supported by the horny fin- 

 rays mentioned on p. 298. The central layer of the thicker parts of the 

 fins is occupied by cartilaginous rays, rods of cartilage, more or less 

 regularly parallel, and most commonly subdivided into segments. In 

 the ventral lobe of the caudal fin these cartilaginous fin-rays are simply 

 the prolonged haemal spines and this hints to us that the cartilaginous 

 rays of the median fin system in general have been evolved out of pro- 

 longed haemal or neural spines. Except in the ventral lobe of the 

 caudal fin, however, the cartilaginous fin-rays are no longer in continuity 

 with the arches of the vertebrae nor do they even correspond with 

 them in number or position. 



In the paired fins the cartilaginous fin-rays are attached at their 

 inner ends to stout basal cartilages (cf. Fig. 164, D), and these in turn 

 are jointed on to the limb-girdle — a stout bar of cartilage embedded in the 

 body-wall which serves as a firm base of attachment for the skeleton 

 of the fin. The pelvic fin is simplified in structure as compared with 

 the pectoral and has only a single large basal cartilage instead of three. 



