BRITISH SP0NGIAD2E". 199 



state of the oscula. When examined by direct light, with 

 a power of about 100 linear, it is seen to be minutely 

 hispid ; but it is very probable that both this character 

 and the corrugations may not exist in the living sponge. 



Although not a single osculum could be detected in an 

 open condition, these organs were readily to be distinguished 

 in the dried specimen by their slightly elevated and rounded 

 margins and depressed areas, and by the projection from 

 the inner side of the marginal ring of a delicate ciliary 

 fringe of spicula, inclining inward at an angle of about 90. 

 When a thin slice of the dermal surface is mounted in 

 Canada balsam, and examined by transmitted light with a 

 power of 160 linear, this ciliary fringe is seen to be pro- 

 duced by the protrusion of the proximal ends of a converging 

 circle of closely packed parallel spicula, of the same size and 

 form as those of the surrounding dermal membrane ; and 

 in the central depressed area the closed membrane of the 

 osculum, with a few retentive spicula only, on its inner 

 surface, is to be seen. The remainder of the spicula of the 

 dermal membrane are dispersed without any approximation 

 to order ; and its internal surface is literally crowded with 

 retentive spicula. 



The skeleton spicula are not nearly so numerous as those 

 of the interstitial membranes. The length of the two is 

 nearly equal, but the latter is not more than half the greatest 

 diameter of the former. Their spination is frequently 

 confined to very near the basal extremity, while in others 

 it extends to about the middle of the shaft of the spiculum. 

 The internal defensive spicula, although of the same form, 

 are readily to be distinguished from the skeleton ones, by 

 their smaller size, their entire spination, and by the much 

 greater comparative size and length of their spines ; both 

 descriptions of spicula frequently appear to be subclavate 

 at the basal extremity, but this character appears to be due 

 rather to profuse spination than to distension of the bases 

 of the spicula. 



The interstitial membranes are profusely furnished with 

 both tension and retentive spicula. The former are 

 frequently congregated in loose and irregularly disposed 



