APPEXDICULAE SKELETON OF VERTEBRATA. 473 



whicli it occupies, it is known as tlie tlioracic (slioulder), or 

 pelvic (hip) girdle. 



Tlie skeleton of tlie free appendage is attached to the extremity 

 of the girdle. When simplest, this is made up of cartilaginous 

 rods (rays), which differ in their size, segmentation, and relation to 

 one another. One of these rays is larger than the rest, and has a 

 number of other rays attached to its sides. I have given the name 

 of Archipterygium to the ground-form of the skeleton, which 

 extends from the limb-bearing girdle into the free appendage. 

 The primary ray is the stem of this ai'chipterygium, the characters 

 of which enable us to follow out the lines of development of the 

 skeleton of the appendage. Cartilaginous arches beset with rays 

 form the branchial skeleton. The form of skeleton of the appen- 

 dages may be compared Avith them ; and we are led to the conclusion 

 that it is possible that they may have been derived from such forms. 

 In the branchial skeleton of the Selachii the cartilaginous bars are 

 beset with simple rays (Fig. 260, a h). In many, a median one is 



a 



Fig. 260. Diagrams to illustrate the homodynamy of the appeudicular skeleton with 

 that of the branchiee. ah cd Branchial arches of Selachii. e Archipterygium. 



developed to a greater size. As the surrounding rays become 

 smaller, and approach the larger one (c), we get an intermediate 

 step towards that arrangement in which the larger median ray carries 

 a few smaller ones (d). This differentiation of one ray, which is 

 thereby raised to a higher grade, may be connected with the primi- 

 tive form of the appendicular skeleton; and, as we compare the 

 girdle with a branchial arch, so we may compare the median ray 

 and its secondary investment of rays with the skeleton of the free 

 appendage. 



We meet with greater difficulties when we come to examine the 

 topographical relations of the appendages. If the comparison of 

 the skeleton of the appendages shows that it is similar to the 

 branchial skeleton, and that therefore it is possible to derive the 

 appendages from branchial arches, we must further suppose that the 

 two appendages were primitively branchial arches, which carried 

 rays, and that they have been differentiated in a different way 

 to the other branchial arches, and have been separated off 

 from the branchial apparatus. The hinder one altered its position 

 more than the anterior one, and this, of course, happened during 

 changes which affected the rest of the organism. The anterior 



