Page:Encyclopædia Britannica, Ninth Edition, v. 22.djvu/438

414 SPONGES of the endoderm into the mesoderm, which, together with the ectoderm, exhibits an independent growth of its own ; and this results in the formation of a thick investment, known as the cortex (fig. 5), to the whole exterior of the Fir.. 5. Ute Argentea, Pol. Part of a transverse section. The concentric circles, indicating transverse sections of spicules, lie within the cortex. After Pole- jaeff, " Challenger" Report ( x 100). sponge. The radial tubes may branch, Heteropegma (fig. 4). If the branches are given off regularly, as the radial tubes were in the first plan, and if at the same time the original radial tube exchanges its flagellated for a pave- ment epithelium, a structure as shown in fig. 6 (Polejna Rliagou type. FIG. 6. Polejna connexiva, Pol. Part of a transverse section. E, excurrent canals, into which the flagellated chambers open. After Polejacff. " Challenger" .Report (x 50). connexiva, Pol.) will result. This form might also be brought about by unequal growth of the gastral endoderm leading to a folding of the inner part of the sponge-wall. Very little direct evidence exists as to which of these two plans has actually been followed. Phylogenetically the transition from a simple Ascon to the most complicated Sycon can be traced step by step; and ontogeny shows that such a Sycon form as Grantia raphanus passes through an Ascon phase in the course of its larval development. Returning to the ancestral form of sponge, Olynthus, let us conceive the endoderm growing out into a number of approximately spherical chambers, each of which com- municates with the exterior by a prosopyle and with the paragastric cavity by a comparatively large aperture, which we may term for distinction an apopyle; at the same time let the endoderm lose ite flagellated character and become converted into a pavement epithelium, except in the spherical chambers. Such a form, called by Haeckel "dyssycus," maybe more briefly named a Rhagon from the grape-like form of its flagellated chambers, which differ from those of a Sycon both by their form and their smaller dimensions. The Rhagon occurs as a stage in the early development of Plakina monolopha (Schulze) and Reniera fertilis (g) (fig. 7) ; a calcareous sponge which appears to Fro. 7. Vertical section of a Rhagon, partly diagrammatic, o, oscule ; p, paragaster. After Keller ( x about 100). approach it somewhat is Leuco2^sis pedunculata, Lfd. By the folding of the wall of a Rhagon, or by its outgrowth into lobes, a complicated structure such as that of Plakina monolopha (20) (see fig. 26 /) results. This is character- FIG. 8. Transverse section across an excurrent canal and surrounding choano- some of Cydomiim eosaster, Soil, e, excurrt-nt canal ; /, flagellated cliambers conimunicating with it by aphodal canals; i, an incurrent canal cut across; s, asterraster ; o, an oxea cut across. After Sollas, " Challenger" Report (x!25). ized by the chambers retaining their immediate communi- cation with the incurrent and excurrent canals, opening into the latter by the widely open apopyle and receiving the former by one or several prosopyles. This may be termed the eury- pylous type of Rhagon canal system. The fold- ing of the sponge -wall may be simple, as in the example given, or too complex to unravel. In -^ higher forms of sponges (Geodinida;, Stelletttdee) ;0 the chambers cease to open abruptly into the excurrent canals : each is prolonged into a narrow canal, ap/iodus, or abitus, which usually directly, sometimes after uniting with one or more of its fellows, opens into an excurrent canal. The prosopyles, now restrict- ed to one for each chamber, may remain unchanged in character, or at the most be prolonged into very short l . R._Dtpiodtocanal system in Cortidum candelabrum, O.S. e, excurrent canal ; the incurrent canal is shown on the left-hand side, near its commencement in the cortex. After F - K Schulze < x 200 >-