56 INVEBTEBBATE MOBPHOLOGT. 



The fact that in some cases both immigration and delamination may occur 

 simultaneously, leading to the formation of a sterrula, bears out the idea 

 that the latter process has arisen from the former. 



Furthermore, it may be pointed out that the occurrence of immigration 

 in such colonial Flagellates as Volvox indicates the primitive character of 

 immigration in the Metazoan blastulas, as well as the manner in which 

 diploblastic organisms have arisen from the more primitive single-layered 

 organisms. 



It is only in the lowest Metazoa, however, that the adult 

 organism is diploblastic. In all others a triploblastic (Fig. 



dm 



sm 



bm 



spia- 



vm. 



Fig. 27. — Diageammatic Tranbveksb Section of an Earthwokm to 

 SHOW THE Triploblastic Condition. 



bm = basemeut membrane. en = endoderm. 



= coelom. sm = somatic mesoderm. 



dm = dorsal mesentery. spm = splanchnic mesoderm. 



ec = ectoderm, tm = ventral mesentery. 



27) condition supervenes during embryonic life, by the devel- 

 opment of a third layer, primitively separated from the endo- 

 derm, and occupying the space which may remain between 

 the two primitive layers. This is the secondary germ-layer 

 or mesoderm. From it there arise the muscular, excretory, 

 circulatory, and reproductive systems in the triploblastic ani- 

 mals, the first and last of these being derived in diploblastic 

 forms from either one or both of the primary layers, while 

 the excretory and circulatory systems are not differentiated. 



The manner of formation of the mesoderm in the embryo 

 varies greatly. In some cases it arises as bilateral pouch-like 

 outgrowths of the archenteron, which later form closed sacks 

 completely surrounding the digestive tract, the sack of either 



