. A text-book of invertebrate morphology. Invertebrates. 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 t
. A text-book of invertebrate morphology. Invertebrates. 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. 0 = 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original McMurri
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