PROTECT/ON, SUPPORT AND MOVEMENT 



507 



tures as horny scales, feathers and hair are sometimes classified as an 

 exoskeleton. 



The skeleton is subdivided into a dermal skeleton consisting of the 

 bony scales and plates mentioned earlier in this chapter, and an endo- 

 skeleton situated beneath the skin. During early embryonic development 

 the endoskeleton is composed of the notochord and cartilage, but the 

 notochord is ephemeral in most vertebrates and cartilage is replaced by 

 bone in most adults. This bone is called cartilage replacement bone to 

 distinguish it from the dermal bone that develops in more superficial 

 parts of the body without any cartilaginous precursor. These types of 

 bone differ only in their mode of development; they are the same his- 

 tologically. 



The endoskeleton and its associated dermal bones can be fiuther 

 subdivided into somatic and visceral skeletons: 



Somatic skeleton (skeleton of the body wall) 



Axial skeleton (vertebral column, ribs, sternum and most of the skull) 



Appendicular skeleton (girdles and limb bones) 

 Visceral skeleton (skeleton of the pharyngeal wall, primitively associated with the 

 gills) 



The Fish Skeleton. The parts of the skeleton can be seen more 

 clearly in a dogfish (Fig. 25.4) than in terrestrial vertebrates. The dogfish 

 skeleton is typical of the skeleton of primitive vertebrates, except that 

 the skeleton is entirely cartilaginous. It will be recalled that the failure 

 of the dogfish's skeleton to ossify is believed to represent the retention of 

 an embryonic condition rather than a primitive adult condition. The 



Centrum-i 

 Neureil arch— i 



Anterior dorsal f in- 



Trunh verl^bra. 

 Spinc- 



Hyomamdlbular- 



Otic Capsule 1 



Chondrocranium 



Naisal capsule 

 PalaVoquadrate 

 (Lower jaw) 



Pdlvic girdle 



Caudal vertebra 



-Pelvic fin 

 Figure 25.4, A lateral view of the skeleton of a dogfish, 



