480 



CHORDATA. 



of the mandibular arch a view which has its opponents, who believe these to 

 be a divided columella 



The tympanic membrane is usually flush with the surrounding skin or only 

 slightly below its level. In the mammals it is protected 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 adorn beneath the vertebral column. The ccelom 

 has degenerated in the 

 head and tail region, and 

 in the amniotes in the 

 neck as well. The cce- 

 lom is, as development 

 shows, an outgrowth from 

 the primitive digestive 

 tract, an enter oavle (pp. 

 147 and 263), lined with 

 epithelium. Since it 

 arises, as in other ccelo- 

 mate animals, by paired 



IT 



FIG. 534. _Fio. 535.^ 



FIG. 534. Ear bones of man (from Wiedersheim). A, incus; H, malleus, S, 

 stapes. 



FIG. 535. Section of vertebrate in abdominal region, a, dorsal aorta; c, ccelom; 

 g, gonad; g/, glomerulus; /, digestive tract; /, liver; m, mesentery; mu, muscular part of 

 mytoomes; mv, its coelom (myoccele); o, omentum; s, spinal cord; so, sp, somatic and 

 splanchnic epithelia; /, nephridial tubule; vm, ventral mesentery, w, Wolifian duct. 



outgrowths from the archenteron, it follows that at firsi the two cavities 

 must be separated by a partition which also encloses the intestinal tract 

 (fig. 535). This partition furnishes the mesentery which usually supports 

 the intestine dorsally in its whole length from the vertebral column, but 

 ventral of the digestive tract only reaches as far back as the liver, so that 

 right and left cceloms unite behind. Some other organs are also sus- 

 pended in the body cavity by membranes: the testes by the mesorc/i him, 

 the ovary by the mesovarlum. In most fishes and in many reptiles the 



