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. ° 



