IX] 



INTRODUCTION TO THE COELOMATA 



135 



A sharp controversy has raged round the question which of 

 these two processes gives us the best representation of what 

 occurred in the evolution of Coelomata from simpler Coelenterata- 

 like ancestors. 



If however we recall the fact that in the Actinozoa the endo- 

 dennic sac has the form of a series of pouches ranged round a 

 central cavity, and that the walls of these pouches become con- 

 verted into muscles and generative cells exactly as does the wall of 

 the coelom, and that pores exist in many cases placing the cavity of 

 these pouches in communication with the outside world, we shall 



FIG. 58. Two stages in the early development of a common fresh-water mollusc,' 

 Planorbis, to show the origin of the rnesoderm cells x 320. From Kabl. 



The ectoderm cells are deeply shaded, the endoderm cells are unshaded. 

 A. Young stage in which the endoderm has not begun to be invaginated ; 

 it is a lateral optical section. B. Older stage, optical section seen in 

 front view ; the endoderm cells are invagiriating, and the two mesoderm 

 cells are seen on each side. 1. Mesoderm or pole-cells ; in B, each has 

 budded off another mesoderm cell. 



be induced to conclude that the coelom was probably evolved from 

 lateral pouches of the gut and that the mesoderm is therefore 

 derived from the primitive endoderm. Where pole-cells occur the 

 cavity of the alimentary canal is small in proportion to the thickness 

 of its wall, and the pole-cell might be looked on as a solid pouch. 



In most Coelomata the mesoderm or coelomic wall forms by far 

 the greatest portion of the body. In the phyla which we have here- 

 tofore considered there are usually, as we have already mentioned, 



