KIBS OF VERTEBEATA. 



439 



All the trimk vertebrae may carry ribs. In Fishes they generally 

 extend as far as the caudal region, without any change in form. 

 They never become united below, for when they are connected with 

 other parts of the skeleton, these parts belong to the dermal skeleton 

 (Clupeidaa). They are rudimentary in the Selachii, and are 

 ordinarily represented by short pieces of cartilage only ; they are 

 larger in the Sturiones (Acipenser). They are not placed on the 

 exact boundary of the ccelorn, but at a certain depth in the muscles ; 

 but this does not affect the explanation that we have given of the 

 origin of the ribs. 



The ribs are better developed in those Ganoids that have a bouy 

 skeleton. In the caudal portion of the vertebral column, as in the 

 Selachii and Sturiones, they gradually pass into the lower arches, 

 which are at first connected with the centra in just the same way as 

 are the true ribs in front of them. 



In the Osseous Fishes the ribs vary very greatly in character. 

 They are frequently rudimen- 

 tary or completely absent 

 (Lophobranchii, Gymnodonta, 

 etc.). It is easy to under- 

 stand, from what has been 

 said above as to the origin 

 of the lower arches in the 

 Teleostei, that these lower 

 arches may carry ribs (Fig. 

 229, 0). In a few divisions 

 of the Physostomi the most 

 anterior ribs undergo changes, 

 for bones connected with the 

 air-bladder are developed 

 from them to form a chain 

 which extends to the auditory 



organ (Cyprinoids). In Polypterus, rib-like structures are placed 

 between the dorsal and ventral muscles, on the side of the trunk, 

 and these extend to the integument. They are found also in Amia 

 and the Physostomi, where they are sometimes so large that they 

 have been regarded as the true ribs. As a rule they are bifurcated 

 at their origin. 



Among the Amphibia the most completely developed ribs are 

 found in the Gymnophiona, where they are found on all but the 

 first and last vertebra. In the Urodela they are rudimentary, and 

 form short pieces, which are movably attached by two processes ; 

 posteriorly they are more simple in form. Just as in the Selachii, 

 they extend into the muscles. The transverse process of the sacral 

 vertebra also carries a rudimentary rib, by which it is connected 

 with the pelvis. In the Anura, likewise, they are rudimentary or 

 completely absent. 



Fi 



229. 



Differences in the arrange- 

 ment of the ribs and transverse processes in 

 the Teleostei. c Centrum, o Upper arches. 

 u Transverse processes, r Bibs. 



