REPORT ON THE HEXACTINELLIDA. 169 



lower surface quite uniformly hollowed out, like the funnel-like superior surface, but is 

 somewhat interrupted by four cruciately disposed radial folds which project inwards 

 (PI. LXVI. figs. 2, 3). 



Towards the lower surface with its groove-like concavity, the cylindrical surface of 

 tlie sides is sharply defined off by a smooth margin with a sharp edge, which projects 

 downwards at a sharp angle. From the u^iper terminal funnel the cylindrical lateral 

 surface is much more irregularly separated, exhibiting manifold inward folds and less of 

 a sharp margin (PL LXVI. figs. 1, 4). 



The whole lower surface, from the trumpet-shaped attachment of the stalk to the 

 sharp-edged bounding margin, is covered by a very smoothly expanded dermal network 

 with fine square meshes. Through this, large round lacunae 10 cm. and more in 

 width can be seen (Pi. LXVI. figs. 2, 3). The character of the cylindrical lateral surface, 

 and of the upper funnel-shaped concavity continued into the central canal of the 

 stalk, differs from that above described in the less uniform development of the bounding 

 membrane, and in its penetration by numerous small and large irregularly disposed 

 round apertures, the latter measuring up to 6 mm. in diameter. These gaps represent 

 the external apertures of numerous efferent canals (PI. LXVI. figs. 1, 4). 



From vertical sections it may be seen that it is only from the inferior grooved surface 

 that the more or less wide afferent canals penetrate upwards into the loose parenchyma 

 of the body, while the corresponding efferent passages all open on the lateral or on the 

 superior funnel-shaped surface (PI. LXVI. fig. 4). From this it follows that not the superior 

 funnel-like surface alone, but the external lateral cylindrical surface also, corresponds to 

 the gastral surface of other Hexactinellids. The true dermal surface, on the other hand, 

 is represented wholly by the groove-like concavity on the lower surface of the body, and 

 by its direct continuation, the outer surface of the stalk. We have thus to do with an 

 evcrsion of the originally inner surface of the cup and with a partial overgrowth of the 

 wall, so that a portion of the originally internal gastral surface, that is to say, the upper 

 portion, has become the external lateral surface, while the whole originally lateral external 

 wall has been confined by the outward and downward curvature of the sharp oscular 

 margin to the inferior concave surface of the present sponge body. This theory of the 

 present shape is in complete harmony with the arrangement of the chamber layer and 

 with the distribution of the siliceous spicules. In a vertical section of the downwardly 

 bent, overhanging, sharp edge of the lateral margin, that is to say, of the original everted 

 oscular rim (PI. LXVIII. fig. 1), it maybe seen that the folded chamber layer is disposed 

 in such a way that all the chambers have their convex external surface turned to the 

 delicately latticed skin which covers the grooved inferior surface of the sponge body. 

 The efferent apertures of the chambers, on the other hand, are all directed towards the 

 cylindrical lateral surface, which therefore corresponds to the gastral surface of other 

 sponges, while the inferior surface is really the dermal. 



(ZOOL. CHALL. EXP. — PART LIII. — 1887.) Ggg 22 



