ir. VERTEBRATA. 



645 



stapes. Most students believe incus and malleus to be parts 

 (quadrate and articulare) of the mandibular arch — a viev/ which 

 has its oj)ponents, who believe these to be a divided columella 

 ffig. 57:). 



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 exteriial 

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

 cartilage, is also confined to the mammals. 



The more imj)ortant 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 enterocode (pp. 109 and 158), lined 

 with ejiithelium. Since it arises, as in other codomate animals, 

 by paired outgrowths from the archenteron, it follows that 

 at first the two cavities must be separated by a partition 



E IG 57R.-Sectioii of vertebrate in abdominal region. (From Kingsley.) o, dorsal 

 acjrta; f, c.elom; (/, i^nnn.!; gl, glomerulus; i, digestive tract; (liver; m. mesen- 

 tery run, muscular part of myotomes; mj/, its co-lom (myocele); "Omentum, 

 ,s, spinalcord; «,,»,,, somatic and splanchnic epithelia; (, nepliridial tubule; l-m, 

 ventral mesentery ; ic, \V(jlftiau duct. 



which also encloses the intestinal tract (lig. 57S). These 

 walls furnish the mesentery which supports the intestine in its 



