264 



ZOOLOGY 



SECT. 



S.SC/i 



sc/b 



FIG. 937. Rana. Diagrammatic transverse 



section through the shoulder-girdle, cor. 



coracoid 

 cavity 



(From 



wards and the mandible very short, in correspondence with the 

 small size of the tadpole's mouth. The quadrate is fused by its 

 pedicle with the trabecular region, the otic process (ot. pr) which 

 unites it with the auditory capsule being formed later. Behind 

 the suspensorium are distinct hyoid (c. hy) and branchial (br. 1 4) 



arches supporting the gills by 

 which the tadpole breathes. As 

 development goes on, the axis 

 of the suspensorium is rotated 

 backwards, producing the wide 

 gape of the adult, and the stout 

 palatopterygoid region of the 

 subocular arch (pal. ptg) gradu- 

 ally assumes the slender propor- 

 tions it has in the adult. The 

 greater part of the hyoid arch 



. ,1 



gives rise to the anterior cornua 



ep. cor. epicoracoid ; gl. glenoid r ,\ -\ >, -i -j 



hu humerus ; scp. scapula ;*. scp. 01 the adult hyoid-apparatus, 



the bod y of which is formed 



from the basi-hyal and basi- 

 branchials, and its posterior cornua probably from the fourth 

 branchial arch. The columella is developed independently, but 

 may perhaps represent a pharyngo-hyal or dorsal segment of the 

 hyoid arch. The stapes is a detached portion of the outer wall of 

 the auditory capsule. Thus, with the assumption of purely aerial 

 respiration, the complex branchial skeleton is reduced to a simple 

 structure for the support of the tongue. 



The shoulder-girdle has essentially the structure already described 

 (p. 81) in general terms as characteristic of the pentadactyle 

 Craniata. The scapula (Fig. 936, S, Fig. 937, scp) is ossified, and 

 is connected by its dorsal edge with a suprascapula (Fig. 933, S. 

 SCP, Fig. 937, s. scp) formed partly of bone, partly of calcined 

 cartilage, and developed from the dorsal region of the embryonic 

 shoulder-girdle. The coracoid (Fig. 936, Co., Fig. 937, cor.) is also 

 ossified, but the procoracoid is represented by a bar of cartilage 

 having an investing bone, the clavicle (Cl), closely applied to it. The 

 suprascapula overlaps the anterior vertebras ; the coracoid and 

 procoracoid are connected ventrally by a cartilage, the epicoracoid 

 (Fig. 936, Co 1 , Fig. 937, ep. cor), which is in close contact with its 

 fellow of the opposite side in the middle ventral line, so that the 

 entire shoulder-girdle (Fig. 937), like that of the Dog-fish, forms a 

 single inverted arch. 



Passing forwards from the anterior ends of the united epicoracoids 

 is a rod of bone, the omosternum (Fig. 936, Ep), tipped by a rounded 

 plate of cartilage, and passing backwards from their posterior ends is 

 a similar but larger bony rod, the sternum (St), also tipped by a 

 cartilaginous plate, to which the name xiphisternum (Kn) is applied. 



