. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. 54 COMPARATIVE ANATOMY Ganoidei, Teleostei, and Dipnoi—In these forms the ribs, ahiiost without exception, are connected with the ventral parts of the notochordal sheath (Dipnoans) or with the " basal processes " (Ganoids and Teleosts, see p. 38).-' This is one point of difference between the ribs of these forms and those of other Vertebrates: another is that they are always situated beneath (internal to) the lateral muscles, between these and the peritoneum (Fig. 40a, A, B, C). In Teleosts the ribs are at fi
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. 54 COMPARATIVE ANATOMY Ganoidei, Teleostei, and Dipnoi—In these forms the ribs, ahiiost without exception, are connected with the ventral parts of the notochordal sheath (Dipnoans) or with the " basal processes " (Ganoids and Teleosts, see p. 38).-' This is one point of difference between the ribs of these forms and those of other Vertebrates: another is that they are always situated beneath (internal to) the lateral muscles, between these and the peritoneum (Fig. 40a, A, B, C). In Teleosts the ribs are at first continuous with the basal processes and become secondarily segmented off from them: this may be a coenogenetic modification. Towards the caudal region, the ribs gradually take on the form of hasmal arches, which have precisely the relations of the ribs as. Fig. 40b.—Antekiok End op the Vertebral Column of PoLrpiERtrs. From the ventral side. WK, centra ; /—F, first five pairs of dorsal ribs.; ft, ventral ribs. described above. In Teleosts, however, the ribs gradually disappear in passing backwards to the tail, and the haemal arches are formed by the basal processes alone (Fig. 40a, B, c). In spite of these differ- ences, however, there can be no doubt that the ribs of Teleosts are homologous with those of Dipnoans and Ganoids. Large rib-like structures (" upper ribs ") are present in Polypterus (Pig. 40b), which have a similar position to that of the ribs in the forms next to be described ; and amongst the Teleostei (Clupeoidei, Salmonidfe) small cartilages are present beneath the lateral line in a similar position to that of the distal The ribs are rudimentary in certain species of all the orders of Fishes, and in some cases their place is taken by fibrous bands, arising from the skeletoce'noiis layer. They are wanting in Cyclostomes. °. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readabilit
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