102 



COMPARATIVE MORPHOLOGY OF VERTEBRATES, 



otic capsule forms the styloid process (p. 100), while the rest may ossify as epi-, 

 cerato-, and hypohyals, or a part may change to a stylohyal ligament, connecting 

 the ventral parts with the skull. The hyoid of the adult consists of the copula 

 forming the body, a part of the hyoid the anterior cornua, while the first branchial 

 arch (of which at most but one or two 'thyrohyaP elements are formed) give rise 

 to the posterior cornua. These are connected by ligament with the greatly modified 

 posterior branchial arches, described in connection with the larynx (see respiratory 

 organs). 



Appendicular Skeleton. 



The appendages fall in two categories, the median or azygos 

 (median fins) found only in aquatic vertebrates and the paired appen- 

 dages, which (cyclostomes excepted) are found in every class, although 

 here and there individual species or genera may lack them. Both 

 kinds have an internal skeleton. Opinions differ as to the origin of 

 these appendages. The two most prominent views are given below. 



FIG. 106. Diagram of the origin of median and paired appendages from lateral fin folds. 



According to one view the two types have no relation to each other. The 

 paired appendages are derived from gill septa, all traces of which are otherwise 

 lost from these somites. The girdles which support the appendages are modified 

 gill arches, while the skeleton of the appendage itself is derived from the radialia 

 which support the gills, one radial forming an axis, the adjacent radials being 

 arranged on either side of this, and carried outward from the arch by the growth 

 of the septum to form the body of the appendage (fig 122). A somewhat similar 

 view is that the appendage itself is a modification of an external gill, such as is 

 found in larval amphibians. 



Another view supposes an ancestor with two pairs of longitudinal folds running 

 the length of the body behind the head, each fold supported by a series of skeletal 

 rods (fig. 106). With farther development the upper folds on either side migrated 



