The porifera and coelentera . un, being usually slightly flattened and spreadout, with an irregular, but more or less circular outline. The uppersurface of the body is studded with minute pores (prosopyles),leading directly into small rounded flagellated chambers, which intheir turn open by wide apopyles into a spacious gastral cavity,lined everywhere by flattened ejjithelium. The water passes outof the gastral cavity by the osculum, which is often raised up likea chimney from the surface of the body. The lower surface of thebody is in contact with the surface of the object to which thesponge
The porifera and coelentera . un, being usually slightly flattened and spreadout, with an irregular, but more or less circular outline. The uppersurface of the body is studded with minute pores (prosopyles),leading directly into small rounded flagellated chambers, which intheir turn open by wide apopyles into a spacious gastral cavity,lined everywhere by flattened ejjithelium. The water passes outof the gastral cavity by the osculum, which is often raised up likea chimney from the surface of the body. The lower surface of thebody is in contact with the surface of the object to which thesponge is attached, and contains no chambers. Hence two regionscan be distinguished conveniently in the body wall; a lower portion,devoid of chambers or pores, tlie hi/poj)hare, and an upper portion,containing all the chambers, the spongophare. From the foregoing it will be seen that the lihagon is con-siderably in advance of the Ohjntlms as regards organisation, sinceit has a canal system of the second tyjjc, with the gastral layer. Fig. S4. Vertical section of a Rhagon, diagrainmatic. o, osculum ; p, gastral cavitv. (Afti-r Keller,X about 100). confined to tlie flagellated chambers, and the gastral cavity linedeveiywheie by flat epithelium of the dermal layer. Xo stage withfully formed pores and osculum, and with a canal system in a stateof functional activity, is known to occur of a simpler type than theRhagon in any Demosponge, but a transitory embryonic stage isoften found which may be interpreted as a suppressed and con-tracted Olynthxis stage (Fig. 63, B). No Demosponge is known, onthe other hand, which remains in the simple lihagon condition ;growth and folding of the wall lead in all cases to a series of pro-gressive complications. The adult type of canal system in Demospongiae isrepresented by such a form as IHakina monolopha (Fig. 61,/), inwhich the upper wall or spongophare of the primitive Ehagon hasbecome folded to form a number of lobes or diverticula. Thefla
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Keywords: ., bookcentury1900, bookdecade1900, booksubje, booksubjectctenophora
