422 RICHARD EVANS. 



true ostia, which are small openings measuring about 15 fx 

 across, and which appear to be intercellular, and not intra- 

 cellular. They occur all over the surface, even close to the 

 osculum (fig. 8). 



The osculum is much larger than an ostium, and is situated 

 near the centre of the young sponge. At first it appears as a 

 funnel-shaped opening, the rim of which is on a level with the 

 general surface. Its sloping sides are perforated by one or 

 more smaller openings which communicate with the exhalant 

 system beneath. The thickened rim which surrounds the 

 osculum soon grows up, and forms a tube carrying the opening 

 at its extreme end. 



Appendix A. 



On the Nutritive Vacuoles and Yolk Bodies. 



The enclosures found chiefly in the cells with vesicular 

 nuclei are of two kinds, and must be described separately. The 

 less numerous and larger enclosures may be termed the " nutri- 

 tive vacuoles," while the smaller and more numerous will be 

 given the name " yolk bodies." 



To have described them fully at the same time as the cells 

 which contain them would have burdened with too much detail 

 the description of the more important elements of the larva, 

 and also would have caused a certain amount of the technique 

 to be mixed up with the description. For these reasons I 

 thought it advisable to describe these enclosures, as well as 

 certain other processes which go on during the development, 

 in a number of appendices. 



(1) The "Nutritive Vacuoles," — These structures are 

 almost invariably restricted — in the free-swimming larva at 

 least — to the cells with vesicular nuclei, which in the earlier 

 stages of the development may contain as many as three or 

 four of them, though one is the rule. Owing to the large size 

 of these cells it often happens that the nucleus and the vacuole 

 must be looked for in different sections. These structures have 

 not been described by previous authors, save as an " occasional 



