8 9 4 



SUB-KINGDOM VERTEBRATA. 



it will be unnecessary to enter here) ; but in others this skeleton 

 remains cartilaginous throughout life, or, as in many Sharks, has 



solid structures formed by calcifications 

 in the cartilage, which are quite distinct 

 from true bone. This endoskeleton may 

 be divided into an axial portion, or that 

 of the head, trunk, and tail ; and an ap- 

 pendicular portion supporting the limbs ; 

 the relations of the two being shown in 

 fig. 814. The anterior part of the axial 

 skeleton is formed by the skull (of which 

 we shall speak later) ; and this is suc- 

 ceeded posteriorly by the vertebral column, 

 typically composed of a number of seg- 

 ments, known as vertebra, placed in the 

 middle line of the dorsal side of the 

 body, and continuing from the head to 

 the extremity of 

 the tail. This 

 column is devel- 

 oped round a rod- 

 like axis known 

 as the notochord. 

 In certain of the 

 lower forms the 

 latter may persist 

 throughout life, 

 and may either 

 simply have a 

 fibrous or carti- 

 laginous sheath, 

 or may have cal- 

 cifications, or car- 

 tilages, in either the dorsal or ventral por- 

 tion ; and there is a complete transition 

 from such a primitive type of column 

 to that of the more specialised types in 

 which the vertebrae are fully ossified. 

 There is an enormous amount of variation 

 in the structure of the vertebrae of differ- 

 ent groups, and even in the different 

 regions of the body of a single animal, 

 but there is one general plan pervading them all. Thus a typical 

 vertebra (fig. 815) consists of a basal portion, or centrum, which 

 may be either disk-like or more or less elongated. Its posterior 



Fig. 815. — Left lateral view 

 of an imperfect amphiccelous 

 dorsal vertebra of a Dicynodont 

 Reptile. sp, Neural spine 

 (broken) ; pz, Prezygapophy- 

 sis ; pt.z, Postzygapophysis ; 

 tp, Transverse process (broken) ; 

 cp, -ffztf-facet on the centrum ; 

 b, b', Muscular ridges. 



Fig. 814. — Under surface of a 

 Labyrinthodont Amphibian (See- 

 Icya), with the dermal armour re- 

 moved from the left side in order 

 to show the endoskeleton. Greatly 

 enlarged. (After Fritsch.) 



