IV. VERTEBRATA. 



545 



stapes. Most students believe incus and malleus to be parts 

 (quadrate and articulare) of the mandibular arch a view which 

 has its opponents, who believe these to be a divided columella 

 (fig. 577). 



The tympanic membrane is usually flush with the surrounding 

 skin or only slightly below its level. In the mammals it is pro- 

 tected by being placed at the bottom of a deep tube, the external 

 auditory meatus. The ear conch, a fold of skin supported by 

 cartilage, is also confined to the mammals. 



The more important vegetative organs of the body are enclosed 

 in a large body cavity or ccelom beneath the vertebral column. 

 This is, as development shows, an outgrowth from the primi- 

 tive digestive tract, an enteroccele (pp. 109 and 158), lined 

 with epithelium. Since it arises, as in other coel ornate animals, 

 by paired outgrowths from the archenteron, it follows that 

 at first the two cavities must be separated by a partition 



PIG. 578. Section of vertebrate in abdominal region. (From Kingsley.) a, dorsal 

 aorta; c, coelom; g, gonad; gl, glomerulus; i, digestive tract; I, liver; w, mesen- 

 tery; rrm, muscular part of myotomes; my, its crelom (myocoele); o, omen turn; 

 s, spinal cord; so, sp, somatic and splanchnic epithelia; t, nephridial tubule; urn, 

 ventral mesentery ; u\ Wolfflan duct. 



which also encloses the intestinal tract (fig. 578). These 

 walls furnish the mesentery which supports the intestine in its 



i 



