The porifera and coelentera . , on the one hand, by furtherfolding of the spongophare, leading to considerable branching andcomplication of both the excurrent and incurrent canals ; and, onthe other hand, by thickenings of, and fusions between, the outerends of the diverticula of the spongophare, with the result, first,that the incurrent spaces become more completely CTiclosed and. cr> o Diagram of a transverse section through tlie outer region of Tetilla pedifero. E, ectosome ;C, choanosoine ; e, excurreut canal; i, incurrent canal; p, ostia. (After Sollas, Challenger Re])orts.) narrowed t
The porifera and coelentera . , on the one hand, by furtherfolding of the spongophare, leading to considerable branching andcomplication of both the excurrent and incurrent canals ; and, onthe other hand, by thickenings of, and fusions between, the outerends of the diverticula of the spongophare, with the result, first,that the incurrent spaces become more completely CTiclosed and. cr> o Diagram of a transverse section through tlie outer region of Tetilla pedifero. E, ectosome ;C, choanosoine ; e, excurreut canal; i, incurrent canal; p, ostia. (After Sollas, Challenger Re])orts.) narrowed to form definite canals; and secondly, that a corticallayer is developed on the external surface of the sponge body. An instructive stage in the evolution of the incurrent systemexhibiting but a slight advance on the state of things found inPlakina monolopha, is seen in the Tetractinellid genus Tetilla (Fig. 85).The dermal layer is greatly thickened at the distal extremity ofeach diverticulum of the spongojihare, and the outer free margin ofeach such thickening is expanded into a rim or plate which uniteswith the margins of other and similar thickenings to form acontinuous dermal membrane, perfectly comparable in its origin tothe pseudoderm often formed in an Ascon colony or the dermalmembrane of some Heterocoela. Over each incurrent canal thedermal membrane is perforated by
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