1 102 COMPARATIVE ANATOMY 



'certain Edentates), are confined to the premaxilla, maxilla, and 

 mandible. 



The hyoid arch (Fig. 80) is connected proximally with the 

 auditory capsule and distally with the base of the third visceral 

 (that is, the first true branchial) arch. It becomes more or 

 less ossified, but the greater part is usually reduced to a fibrous 

 band, and may be quite rudimentary; its proximal end forms 

 the styloid process of the periotic, and its distal the lesser 

 (anterior) cornua of the so-called hyoid bone of the adult. The 

 body of this bone represents the basal parts of the hyoid and 

 first branchial arch, the greater (posterior) cornua belonging to 

 the latter. 



VI. LIMBS 



The problem of the origin and morphological meaning of the 

 fins and limbs of Vertebrates is one which, in point of interest and 

 importance, is comparable to that relating to the head. During 

 the last thirty years it has been attacked vigorously both from the 

 embryological and the palaeontological sides, and has given rise to 

 so many speculations often of a very contradictory nature that 

 only the barest outline of some of the more important results 

 obtained can be given in the course of the present chapter. 



The fins or limbs, which are distinguished from the axial organs 

 (head, neck, and body), as appendicular organs, serve mainly for 

 locomotion, and may be divided into two groups, the unpaired 

 and impaired (pectoral and pelvic). 



A. Unpaired Fins. 



The unpaired, or median fins, which are mainly characteristic 

 of Fishes, arise in the embryo as a ridge of the integument (epiblast 

 and mesoblast) extending along the median dorsal line from the 

 anterior part of the trunk backwards to the tail, around the apex 

 of which it is continued forwards for some distance along the 

 ventral side : thus a dorsal, caudal, and ventral portion can be 

 distinguished. In the course of further development, these 

 portions either remain continuous, or else certain parts undergo 

 reduction, so that the ridge only persists in certain regions, 

 where it forms independent dorsal, caudal, and ventral or anal fins 

 (Fig. 81, A, B) : in these regions muscles and skeletal parts be- 

 come developed. 1 



These skeletal parts consist of supporting rays of two kinds. 

 In the base of the fin cartilaginous radii, usually segmented, are 



1 The curious suctorial disc on the dorsal side of the head of the Teleostean 

 Remora (Echeneis), by means of which it attaches itself to foreign objects, arises 

 in the embryo from the anterior portion of the dorsal unpaired fin, and this is 

 indicated throughout life by the arrangement of the blood-vessels, nerves, and 

 skeletal parts. 



