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neraa , Acauthascus. In the single genus Hyalonema we find al- 

 ready a considerable diversity of forms. *) In all the Hexactiuel- 

 lids here mentioued the apopyles are all directed towards the cen- 

 trum of the sponge. The inhalaut apertures are on the convex 

 outside, the exhalant ones on the concave inside. And this as 

 well in those with a narrow cylindrical tube, as in those with 

 a wide cup or funnel, There is for the moment not one reason 

 which contradicts the suggestion that the concave side of the cup 

 is homologous to the cylindrical tube of others , and is thus also 

 to be regarded as a cloaca , the opening as an osculum. 



In some of these cup-shaped sponges the walls are thin , in 

 others they are rather thick in coraparison to the general size 

 of the animal. In some , the upper part . i. e. near to the oscu- 

 lum , is thin , whereas lower down the wall is of considerable 

 thickness. In extremes we then find a massive cone, cylinder or 

 sphere with a more or less shallow depression. And this shallow 

 depression is thus of course the aequivalent of a deeper cup or 

 cylindrical tube. In fact, in such sponges the exhalant apertures 

 lie congregated in the depression , shallow as it may be , and 

 only lie there. Now we know sponges where this depression varies 

 a good deal aud may even hardly be recognisable. This we see 

 in Synops. Still , the shallow depression corresponds to the cloaca 

 of other sponges and the ultimate exhalant apertures of Synops 

 therefore are not oscula hut procts. And if we then see that in 

 Isops there are exactly the same sort of apertures on the surface 

 of the spherical sponge, hut that there is no depression at all 

 here, we may fairly well assume that those openings of the ex- 

 halant choues are also procts and not oscula. The same is true 

 for Caminus. Only we find here that the cortex around the proct 

 is exceedingly developed. In Geodia there is again a cloaca in 

 which the procts debouch very regularly. Whereas in Cydonium , 

 where Ave sometimes find a shallow depression sometimes a dee- 

 per , cylindrical one , the cloacal wall is built up exactly as the 



*) See the pictures of Schulze (Challenger Keport.) 



