. Elements of Comparative Anatomy. . RIBS OF VERTEBEATA. 439 All tlie truuk vertebrce may carry ribs. In Fishes tliey generally extend as far as tlie 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 (Clupeidas). 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 coelom, but at a certain depth in the muscles; but this doe
. Elements of Comparative Anatomy. . RIBS OF VERTEBEATA. 439 All tlie truuk vertebrce may carry ribs. In Fishes tliey generally extend as far as tlie 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 (Clupeidas). 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 coelom, 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 bony 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- taiy 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 r
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