Characteristics of the Vertebrata. Ixix 



and in the presence of a largely developed branchial apparatus, 

 which is similarly correlated with their aquatic life. In the Mar- 

 sijjobiruicjiii, vertebrae are indicated rather than ditf'erentiated by 

 the development of a few cartilaginous neural and haemal arches, 

 the sheath of the chorda dorsalis remaining unsegmented through- 

 out. Indications of the formation of vertebral centra are presented 

 to us in the calcified annuli developed in the sheath of the chorda 

 in Chimuerae. The characters of the axial elements of the endo- 

 skeleton vary much in Plagiostuini, attaining in some represen- 

 tatives of this sub-order to perfect differentiation and partial calci- 

 fication. Greater variety is observable in the same structures in 

 the now numerically much smaller order of Ganoidei, where the 

 centra may be represented by a cylindrical fibro-cartilaginous sheath 

 surrounding the cylindrical notcchord, as in the Sturgeons ; or by 

 perfectly ossified opisthocoelian masses connected by anchylosis 

 with perfectly ossified neural arches, as in the Bony Pikes {Lejii- 

 dodeidae). In Teleostei, as the name implies, the vertebrae are 

 differentiated, and, in various degrees, calcified, the amount of lime 

 deposited rarely or never attaining the proportions it assumes in 

 other classes of Vertebrata. The neural arches are in Teleostei 

 ordinarily, but not always, anchylosed to the centra, without the 

 interposition of any neuro-central suture. The number of the ver- 

 tebrae may be as many as 365 in some Sharks ; in some Ganoids, 

 and in some of the Phj/sostoini amongst Teleostei, it may amount 

 to 200 ; in most PliJ/sostomi it is about 80 ; it falls much lower in 

 Acaiithojjteri, and may be as low as 15 in the Plecfognaihi. The 

 trunk is divisible into two main regions, the dorsal and the caudal ; 

 from the former of which a cervical regnon may be said to be marked 

 off, at least morphologically, inasmuch as the scapular arch makes its 

 first appearance opposite the interval between the second and third 

 vertebrae. The Elasiiiohmnchii and most of the Ganoidei, have 

 their greater geological antiquity spoken to by their retention of 

 the more typical heterocercal form of the tail. This peculiar shape 

 is produced by a disproportionate development of the haemal caudal 

 arches, whereby the tail, which was in the early embr\o equilobed, 

 and, as in Marsipobranchii and Dijnioi, a direct continuation of the 

 axis of the dorsal region, is bent upwards. An additional factor in 

 the production of the heterocercal tail, is brought into play in the 

 case of the Holostean Ganoidei, and the Physostomous Teleostei^ 



