38 



COMPARATIVE ANATOMY. 



The caudal region of the vertebral column deserves particular 

 attention in Fishes, and the primitive condition of this region in 

 Amphioxus, Cyclostomi, and Dipnoi, may be taken as a starting- 

 point. In these, the notochord extends straight backwards to the 

 hinder end of the body, and is surrounded quite symmetrically 



FK 



FIG. 27. PORTION OF THE VERTEBRAL COLUMN OF A YOUNG DOGFISH (Scyllium 

 caniculd). (After Car tier.) 



G) notochord ; Kn> outer, and JTw 1 , inner, zone of cartilage ; FK, the fibre-carti- 

 laginous mass lying between these zones, which is undergoing calcification ; Li, 

 intervertebral ligament. 



by the tail-fin, which is therefore spoken of as protocercal or 

 diphy cereal (see Fig. 29). This condition is met with also in 

 many Wishes of the Devonian strata, as well as in young stages of 

 Teleostei. In the latter, however, the ventral half of the tail-fin 

 with its supporting skeleton becomes, as a result of unequal growth, 



FIG. 28. PORTION OF THE VERTEBRAL COLUMN OF Scymnus. 



WK, centra ; Ob, upper arches ; Ic, intercalary pieces. The apertures for the spinal 



nerves are seen in the arches and intercalary pieces. 



more strongly developed than the dorsal, and thus the vertebral 

 column is bent up dorsally, giving rise to a heterocercal tail. 

 This form of tail may be recognised externally, as in many Elasmo- 

 branchs, Ganoids, and numerous fossil Fishes, or may be masked by 

 a more or less symmetrical tail-fin, when it is only visible internally 

 (Lepidosteus (Fig. 30) and Amia to some extent, but more particu- 

 larly in most Teleosteans, 1 e.g. Salmo, Esox). 



1 The term homocercal is sometimes used to describe the masked heterocercal 

 condition of the tail in Teleostei. 



