. Elements of comparative anatomy. Anatomy, Comparative. ALIMENTARY CANAL OF C(ELENTEEATA. Ill known; just as uncertain is that of the two ciliated surfaces, on either side of this vesicle—the polar areas, which are surrounded by short fringe-like processes. Alimentary Canal. § 86. With the separation of the body into an ectodermal and an endodermal layer, we get the lowest condition of the organs of nutrition; the endoderm investing a space which is open to the exterior, and which is the earliest distinct digestive cavity (stomach, euteron)—(cf. supra, § 28). This condition is simplest in the
. Elements of comparative anatomy. Anatomy, Comparative. ALIMENTARY CANAL OF C(ELENTEEATA. Ill known; just as uncertain is that of the two ciliated surfaces, on either side of this vesicle—the polar areas, which are surrounded by short fringe-like processes. Alimentary Canal. § 86. With the separation of the body into an ectodermal and an endodermal layer, we get the lowest condition of the organs of nutrition; the endoderm investing a space which is open to the exterior, and which is the earliest distinct digestive cavity (stomach, euteron)—(cf. supra, § 28). This condition is simplest in the Gastrula form, and undergoes various differentiations in the two chief divisions of the Coelenterata. The stomachal cavity, that is, does not remain a mere simple space, but grows out into various kinds of cavities, canals, pouches, and so on, which are either distributed irregularly in the organism, or are arranged in a definite way. As a rule, division of labour takes place at the same time, and only one definite part, or several such parts, functions as a digestive cavity, while the other spaces are used to distribute the nutrient fluid (chyme). But this gastric system has other functions too. There is no doubt that it also has a respiratory function, by distributing through the body the water that was taken in with the food; and for this purpose it has, especially in the Spongite, a much more extended surface than has the outside of the body. Finally, it has important relations to reproduction, for the generative elements are formed in its walls. § 87. Among the SpongiaB the simplest form is limited to the early stages of development, or is per- manent as in the Gastrasades. Almost the only complication in the Gastrasades is the de- velopment of an arrangement for producing a current at the entrance of the simple enteric tube. In the Porifera there are various new complications. Temporary spaces appear in the endodermal layer, which break through to the exterior, so
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