22 



NATURAL HISTORY OF VERTEBRATES. 



similar restriction of primitively continuous right and left lateral folds to two tracts, 

 an anterior and posterior on either side. These tracts, which are in their most primi- 

 tive condition in certain sharks, have given rise to the paired fins of other fishes, and to 

 the anterior and posterior extremities of the higher vertebrates. Like the unpaired 

 fins they are supported by fin-rays (actinosts) and supporting bones (actinophores), but 

 the latter do not enter into direct union with the vertebral column, although it is prob- 

 able that they have a metameric significance here as well as there. The actino- 

 phores of both anterior and posterior fins acquire independent supports, which in 

 the sharks are merely two unsegmented arches of cartilage whose convexities lie on 

 the ventral surface of the body, but which give rise to the pectoral and pelvic 

 arches of higher vertebrates. 



It is comparatively easy to understand how the skeleton of the anterior and pos- 

 terior fins of a shark-like form might be converted into that of a bony fish, but it is 

 somewhat more difficult to understand how the limb skeleton of the higher vertebrate 

 is comparable therewith. Nevertheless the condition in the shark forms the best start- 

 ing point for the discussion of the paired limbs 

 of the vertebrate branch. What this is may be 

 seen from Fig. 27. The cartilaginous pectoral 

 arch is composed of a right and left half inti- 

 mately united in the ventral middle line. Half 

 way up, three basal cartilages are attached to it 

 on either side, supporting the cartilaginous rays, 

 which again run out into the finer thread-like 

 rays of the fin. The basal cartilages are known 

 as pro-, meso-, and meta-pterygium, but it is 

 to be noted that the propterygium is seconda- 

 rily dorsal, not anterior, while the metaptery- 

 gium is ventral, not posterior in position. This 

 implies a rotation of the anterior edge of the 

 fin through an angle of 90°. Most of the rays are attached to the metapterygium, 

 very few being on its ventral (primitively posterior) edge. The pro- and meso-ptery- 

 gium may be regarded as rays which have lost their attachment to the metaptery- 

 gium, and have acquired new importance by attaching themselves directly to the 

 pectoral arch, while the metapterygium forms the principal axis, giving origin to 

 rays which are chiefly attached to its anterior border. The posterior fins of the 

 sharks are essentially constructed on the same type as the anterior, although they 

 are decidedly less altered from the primitive condition. 



In respect of the structure of the limbs as well as in other anatomical features, 

 the sturgeons are intermediate between the sharks and the bony fishes. The latter are 

 characterized by the reduction of the parts corresponding to the above described car- 

 tilages, which results from superficial bones developed in the skin, having largely 

 usurped the function of these. This remark is especially applicable to the pectoral 

 arch,, the investing bones of which (developed in part round that portion of the neuro- 

 mastic tract which ascends from the lateral line to the head) constitute the great bulk 

 of the shoulder-girdle in these forms. 



It is very much otherwise in the higher vertebrates. In these the primary cartila- 

 ginous structures persist as the pectoral and pelvic girdles, which we may now shortly 

 consider in the turtles, whose limbs retain many primitive features, as a convenient 



Fio. 27. — Pectoral girdle of a shark t^Scyllium) ; 

 cr, coracoid ; ma, mesopterygium ; mt^ me- 

 tapterygium ; p, protopterygium ; r, radial car- 



