SKELETON 



125 



copulation and is supported by the specialized terminal radialia of the 

 metapterygium 



In other ganoids and in teleosts the skeletal parts are more or less 

 ossified, the basalia more numerous than in the higher elasmobranchs 

 and are shortened and more closely associated with the girdles, while 

 the numerous radii form most of the skeleton of the fin itself. It is 

 not uncommon for the anterior element of the pectoral fin to form a 

 strong defensive spine, not infrequently connected with a poison 

 gland (fig. 23). In some teleosts, e.g., eels, the pelvic fin may be 

 lacking. The fins of the dipnoi are easily understood by comparison 

 with a biserial fin like that of Xena- 

 canthus (fig. 135). The axial part 

 has been elongated and in Ceratodus 

 it bears biserial radialia, while in 

 Protopterus and Lepidosiren only the 

 axis persists. 



Embryology tells little as to the primitive 

 condition of the ichthyopterygium, for in the 

 procartilage stage the condensation of 

 mesenchyme for the skeleton of the fin forms 

 a continuum which later becomes broken into 

 the separate parts (fig. 136). 



The legs (chiropterygia) of all 

 tetrapoda are essentially alike (fig. 



119). Each consists of several Fig. i36.-Cartilage skeleton of 

 regions, comparable in detail with shoulder girdle and left pectoral fin of 

 -,»«u *u T-u • 1 • ^L larval Po/y/>/crM5, after Budgett, ic/, 



each other. The proximal is the foramina for blood-vessels; c, cora- 



Upper arm (brachium) or thigh ^°*^' ^/' coracoid foramen; mes, 

 fr \ . . . , mesopterygium; wc^ metapterygium; 



(femur) containing a single bone, the pro, protopterygium; r, developing 



humerus or femur in the fore and '^"^'^''^^ '' '"^P"'*' 

 hind limb respectively. The next region, the forearm (ante- 

 brachium) or shank (cms), contains two bones, a radius or tibia on 

 the preaxial and an ulna or fibula on the postaxial side. Next follows 

 the podium, the hand (manus) in front, the foot (pes) behind, each 

 consisting of three portions. The basal podial region, the wrist 

 (carpus) or ankle (tarsus) consists of several small bones; the second 

 division (metapodium) is the palm (metacarpus) or instep (meta- 

 larus) and lastly come the fingers or toes (digits), each digit consisting 

 of several bones, the phalanges. These separate parts are included 

 in the accompanying table, in which the terms given to the separate 

 elements of the wrist and ankle of man are included. 



