. Outlines of zoology. Zoology. MORE COMPLICATED FORMS. rlS sist of—(i) a flat ectoderm ; (2) a mesogloea containing triradiate calcareous spicules, phagocytes, and reproductive elements; and (3) an endoderm lining the central cavity, and composed of collared flagellate cells, almost exactly like some of the monad Infusorians. This simple sponge is not much above the gastrula level; it agrees generally with a simple Ccelenterate, such as Hydra, but differs from it in the absence of tentacles and stinging cells, and in the greater development of the mesogloea. More complicated forms.—But a desc
. Outlines of zoology. Zoology. MORE COMPLICATED FORMS. rlS sist of—(i) a flat ectoderm ; (2) a mesogloea containing triradiate calcareous spicules, phagocytes, and reproductive elements; and (3) an endoderm lining the central cavity, and composed of collared flagellate cells, almost exactly like some of the monad Infusorians. This simple sponge is not much above the gastrula level; it agrees generally with a simple Ccelenterate, such as Hydra, but differs from it in the absence of tentacles and stinging cells, and in the greater development of the mesogloea. More complicated forms.—But a description of a simple sponge like Ascetta conveys little idea of the structure of a complex form such as the bath-sponge (Euspongia). Let us consider the origin of complications. (a) Sponges—long regarded as plants — are plant - like in being sedentary and passive. They seem also to feed easily and well. Like plants, they form buds, the outcome of surplus nourishment. These buds, like the suckers of a rose- bush, often acquire some apparent independence, and the sponge looks like many vases, not like one. More- over, as they grow these buds may fuse, like the branches of a tree tied closely together. Thus the structure becomes more intricate. (b) In the simple sponge the gastric cavity of the vase is com- pletely lined by the collared endoderm cells {Ascon type). But the endoderm may grow out into radial chambers, and the walls of these may also be folded into side aisles (Sycon type). The outgrowing of the endoderm into the mesogloea may be continued even further, and the cells may become pavement-like, except in the minute flagellate chambers, where the characteristic collared type is retained {Leucon type). (See Fig. 50.) [Speculatively, it may be suggested that the characteristic folding or outgrowth of the endoderm is necessitated by the. Fig. 49.—Section of a sponge.—After F. E. Schulze. Showing inhalant canals, flagellate chambers, a gastrula forming in the mes
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
