536 



STRUCTURE OF VERTEBRATA 



There are three pro-otic head-segments (pre-mandibular, mandibular, 

 and hyoid), which correspond to the orbital region, their walls forming 

 the six eye-muscles. Behind the auditory capsule there are ten or 

 eleven head-segments. 



Vertebral column. — A dorsal skeletal axis is character- 

 istic of Vertebrata, and its usefulness is evident. It gives 



Fig. 303. — Diagrammatic section through a Vertebrate animal. 



Ect., Ectoderm, the epidermis ; /, somatic layer of mesoderm, the 

 dermis ; //, the ccelom or body cavity ; SPL., splanchnic 

 mesoderm, enveloping the endoderm-lined gut (/V) ; ///, 

 the endoderm lining of the gut ; VI, the notochord folded 

 off from the dorso-median line of the gut ; V, the nerve- 

 cord, insunk from the ectoderm. 



coherent strength to the body ; it is usually associated very 

 closely with a skull, with limb girdles, and with ribs ; it 

 affords stable insertion to muscles ; its dorsal parts usually 

 form a protective arch around the spinal cord. 



