106 I. IJIMA : HEXACTINELLIDA. I. 



thickening of the wall in the presence of interruptions in the 

 form of parietal oscula. 



In somewhat older specimens of 32-3Ô mm. length and 10-11 

 mm. greatest breadth, there already exist a varying number of 

 parietal oscula, which are still very small but yet visible as distinct 

 openings. They first break through in the bulged middle portion 

 of the body. Nearer the ends but especially in the upper region, 

 they are still in a state of incipient formation, being covered over 

 by the dermal layer. The wall-tissue between the open parietal 

 oscula has begun to swell out gently as the first step in the 

 formation of the ledges, which henceforth become more and more 

 conspicuous as the sponge advances in growth. Fig. 7, PI. lY, 

 shows the skeleton of an individual of the size in question, from 

 which the loose tissues have been rubbed off. 



As representatives of still larger young specimens, whose 

 appearance has notably approached that of adults, will serve the 

 two shown in figs. 8 and 9, PI. IV. In the smaller specimen 

 (fig. 8) the parietal oscula, though yet small in size, are already 

 numerously present. They may be said to be situated at the bottom 

 of shallow dimple-like depressions of the external surface. In 

 many of the depressions, however, the oscula have not yet opened 

 through. In the larger specimen (fig. 9), the externaV elevations 

 between the parietal oscula have definitely taken the form of an 

 irregular network of ridges or ledges, which are most pronounced 

 on the broadest middle portion of the body. Each depressed 

 mesh generally contains only a single oscular opening, but some- 

 times there are more in an indefinite arrangement. The largest 

 of the openings are still under 1 mm. in diameter. They occur 



