SILICIOUS AND HORNY SPONGES WILSON. 315 



Doubtless most of the apertures are closed, but there is a sufficient 

 number of open ones to show with some certainty the specific arrange- 

 ment. The following data bear upon the specific arrangement. 



The surface shows irregular, meandering, interconnected tracts, 

 immediately below which sterrasters are absent (pi. 46, fig. 8). It 

 is in such tracts, which quite lack definite boundaries, that the open 

 pores are found. These are either well apart, in which case doubt- 

 less most of them are simply closed ; or they are close together, sepa- 

 rated by about the width, or by less than the width, of a pore. The 

 latter condition perhaps shows the natural state of the surface, when 

 the sponge is expanded and the pores are all open. At any rate, 

 the open pores and groups of open pores are scattered in such a 

 wide and general way as to indicate that possibly the dermal mem- 

 brane is uniformly perforated (or rather perforable) with pores, 

 that is, that there are no well defined pore areas and aporous tracts. 



Vertical sections show that the sterraster-free areas just referred 

 to (fig. 2) are largely occupied by subdermal cavities of some size. 

 The thin dermal membrane roofing over these is pierced by the 

 pores, which thus open directly into the cavities. Such a region of 

 subdermal cavities connects below with endochones. Laterally such 

 a region fades away into a thin ectochrote, which over the bulk of 

 the sterrastral layer consists, in the actual specimen, of only the 

 thin aster bearing, dermal membrane. 



Endochonal canals of the usual type, narrow radial canals about 

 250 jjl in diameter, perforate the sterrastral layer. The contracted 

 inner ends of the chones form dense, subcorneal, little masses project- 

 ing into the subcortical spaces; scattered over the inner face of the 

 cortex, 1-2 mm. apart. The contracted inner ends of the chones are 

 to be seen in vertical sections and in ordinary flat preparations; 

 their distribution over the inner face of the cortex can also very 

 conveniently be studied in dried pieces from which the choanosome 

 has been picked away. 



The megascleres have the usual arrangement. There are oxeas 

 abundantly scattered through the choanosome. In the periphery 

 of the choanosome are numerous radial bundles, some of the 

 megascleres of which enter and even pass through the cortex. The 

 cladomes of the plagiotriaenes, for instance, are characteristically 

 in the ectochrote. 



Spicules. — 1. Oxea, 2.5-3.5 mm. long, 44-48 p, thick, with many 

 smaller sizes ; in the radial bundles and scattered in the choanosome. 

 A long, slender type, 20 p. thick and less, occurs among the spicules 

 which conspicuously project from the surface. 



2. Plagiotriaene (pi. 46, figs. 7a, 8). Rhabdome long, 50 \l thick 

 near cladome; clads 175 (jl by 50 [x. Smaller ones with clads down 



