SPONGES. 117 



thereabouts, of the above-mentioned sand-grains, between which there are also 

 numerous styli, arranged, partly at any rate, in irregular bundles, and various forms 

 of asters. The upper third of the thickness of the disk is free from sand, and 

 includes a considerable thickness of the choanosome as well as the cortex. Here the 

 main skeleton is formed by stout bundles of styli running vertically to the surface 

 and entering the tubercles, from which they project as dermal brushes (fig. 2, d, b). 

 These vertical bundles of styli are crossed at right angles by similar bundles which 

 lie near the inner limit of the sand-free layer of the choanosome and terminate in the 

 feebly-developed spicular fringe (m.f.) at the margin of the disk. 



The cortex (figs. 2, 3, cort.) is strengthened by an immense number of asters of 

 various shapes and sizes, forming a very dense skeleton, especially towards the surface. 

 Asters of various kinds are also very abundantly scattered through all parts of the 

 choanosome. 



Spicules. (1.) Styli (Plate VI., fig. 4) ; long, slender, nearly straight, evenly 

 rounded off at the base and tapering very gradually to the apex. The size of these 

 spicules is so variable that it seems almost useless to give measurements. The largest 

 are so long that it is difficult to get them unbroken ; 17 millims. by 0'012 millim. is 

 perhaps a fair average size for the full-grown spicule, but considerably stouter (and 

 presumably also longer) examples frequently occur (broken) in boiled-out preparations, 

 while the styli which fringe the edges of the pore-bearing grooves, for example, are, 

 on the other hand, very much smaller than the measurements given above. 



(2.) Euasters (Plate VI., figs. 5, 6); enormously abundant and varying so greatly 

 in form and size that it is impossible to separate the different kinds sharply from one 

 another. The following may, however, be regarded as the principal types : 



(a.) Spherasters with very small centrum and long, conical, stout, sharply and 

 gradually pointed, often slightly curved actines, about 1 1 in number ; total diameter 

 of spicule about O'l millim., with centrum 0'02 millim. in diameter and actines 

 0'044 millim. long. These asters pass gradually on the one hand into smaller 

 oxyasters, and on the other into larger forms with very irregularly curved and more 

 or less (often much) branched actines ; the most copiously branched forms appear to 

 be characteristic of the sandy layer of the choanosome, where they may attain a total 

 diameter of as much as 1 8 millim. 



(6.) Minute chiasters, with about 8 fairly stout, sub-cylindrical, tylote actines; 

 total diameter about O'OOS millim. 



(c.) Oxyasters or spherasters, with small centrum and about 11 slender, conical, 

 oxeote, minutely spined actines ; total diameter about 0'02 millim. The actines are 

 occasionally branched. 



(d.) Similar to (c.), but with tylote actines; this is perhaps the most uncommon 

 form of the aster. 



After carefully removing the sand-grains from below, it is possible to prepare 

 microtome sections of the outer portion of the sponge, including the cortex and that 



