C@LOM. I21I 
walls into epimere, mesomere and hypomere, the ccelom undergoes a 
corresponding division. That portion in the epimere is divided into 
a series of cavities in the myotomes (myoceeles), which are eventually 
obliterated (p. 126); the portions in the mesomere persist only as the 
lumina of the excretory organs and their ducts, described under the 
urogenital system; while that part of the original ccelom in the hypomere 
gives rise to all of the permanent body cavities of the adult. 
The hypomeres gradually descend between the ectoderm and the 
entoderm (fig. 130) until their lower margins meet, ventral to the diges- 
. tive tract. In this way the latter 
becomes surrounded by a pair of 
cavities, the splanchnoceeles or body 
cavities of the adult. Each is 
bounded by epithelium, the tunica 
serosa, in which an outer or somatic 
wall is turned toward the ectoderm, 
while the inner or splanchnic wall 
adjoins the alimentary canal. Later, 
when the muscle plates extend down- 
ward (fig. 135), they unite ectoderm 
and serosa into the outer body wall, 
the somatopleure, while the invasion 
of mesenchyme unites the splanchnic , ee . henoties 
serosa with the entoderm into a similar hypo-, and mesomeres, the walls of the 
ceelom, dm, vm, to form the dorsal 
splanchnopleure. and ventral mesenteries. 4, alimen- 
Mesenteries.—As has just been ‘ty canal; ¢, ectoderm; so, sp, soma- 
stated the walls of the two coelomic ie ea ee 
cavities meet below the digestive tract, thus forming a double membrane 
running lengthwise of the body and binding the alimentary canal to the 
ventral body wall. This membrane is called the ventral mesentery. 
In a similar way the splanchnic walls meet above the digestive tract 
forming a dorsal mesentery. ‘These mesenteries are eventually more 
than double serosal walls, since mesenchyme comes in between, uniting. 
them and affording a tissue through which blood-vessels, lymphatic 
vessels and nerves can reach the digestive organs. 
For convenience of reference different parts of these mesenteries have received 
special names, according to the organs supported. The ventral mesentery 
usually almost entirely disappears, only a small portion persisting in the region 
of the liver, the mesohepar, which, in the ichthyopsida may carry blood-vessels 
