78 THE SPONGES. 



absent, and the surface has a homogeneous appearance, which is perhaps 

 the earlier ontogenetically. The pores, which vary considerably in size, 

 are thickly scattered over the whole dermal surface, not only directly over 

 the conspicuous afferent canals, but in the regions between such, where 

 the dictyonal framework and the flagellated chambers closely approach the 

 surface (Fig. 7, Plate 11). 



A small tubularian hydroid, the polyps of which have four tentacles 

 and are borne upon a slender, ramifying stolon, is present in considerable 

 abundance in the tissues of this sponge. The polyps lie especially in the 

 j)eripheral region, and cause no elevations or apparent malformations. 



The tissues also contain (Fig. 7, Plate 11) exceedingly numerous roimded 

 or irregularly shaped masses up to 120 /a in diameter. These are frequently 

 nearly or quite in contact, so abundant are they. Their histological condi- 

 tion does not admit of exact study, but it may be seen that they are com- 

 posed of closely packed rounded bodies about 4 fx in diameter. They are 

 doubtless collections of archaeocytes such as Ijima has recently described 

 (Ijima, 1901). 



It is difficult to carry out in this sponge the distinction between a 

 dermal surface and a gastral surface. If we do adhere to the latter con- 

 ception, it is obviously only the walls of the cloaca-like main canals which 

 represent the gastral surface. But such walls are of course something 

 quite different from the inner surface of a cup-shaped plate, such as, for 

 instance, in Hexactinella veniilabriim is usually denominated the gastral 

 surface. 



The dictyonal framework is fine, far more so, for instance, than that 

 of H. vcntilahrum F. E. Sch. On parts of the surface the reticulate plates 

 characteristic of the genus are obvious, their superficial edges being not 

 at all or scarcely united. Elsewhere the superficial edges of the plates 

 are united by skeletal strands. In this way a surface network is pro- 

 duced, enclosing rounded or elongated apertures and obscuring the system 

 of plates. The reticulate plates are something less than 1 mm. to 1.5 nun. 

 wide at the surface, and 1 mm. or less apart. They taper internally to 

 an apex, meeting and fusing with one another, as they get farther from 

 the surface. They are of varying thickness, in places consisting of but 

 a single layer of beams, generally of a few (2 to 4) such layers. The 

 individuality of the skeletal plates, both at and internal to the surface, is 

 far more distinct in certain regions than in others, depending on the extent 



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