38 



< < >M 1'ARATIVE ANA'H >M Y. 



The caudal region of the vertebral column deserves particular 

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

 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 VERTF.I:I:AL COLUMN OF A YOUNG DOGFISH 



caniculn). (After Cartier.) 

 Q notochord ; Kn, outer, and A"// 1 , inner, zone of cartilage ; FK, the filim-enrti- 

 lugiimus muss lying between these zones, which is undergoing calcification; Li, 



intorvcrtrliral ligament. 



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

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

 ninny Fishes 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, 



KM:. "J.X. --I'r.KTlDN UK Tlir. V I i: I I I'.i: \ I. COLUMN OF ScyiMVUA, 



ll'/\, nii.i ; 1 1/'. upper arches ; /, int. ivalary pi^-cs. The apertures fyr the spinal 



ii.Tvr.-, ;uv sin ii in the arches and intercalary pieces. 



more stnin.Jv developed tli;m the dorsal, and thus the vertebral 

 rcliimn is limt ii]> dorsally, giving rise to a heterocercal tail. 

 Tins I'.M-III ..(' lull maybe rccogniseti cxttM-n.-illy, as in many Eliismo- 

 branchs, C:ui.)ids. ;md numerous fossil Fishes, or may be masked by 

 ; , moreor less symmetric! tail-tin, \vln-ii it is only visible internally 

 rL-pi.l..>t.-u^ l''u. :in and Amia to some extent, but more particu- 

 larly in in.-i Teleosteans, 3 <.;/. Salmo, Esox). 



i Tli.'t.nii li<>i ..... :ercal |A sometimes nsei the masked hotorocercaJ 



nf tin- 



n 



