290 Mr. W. J. Sollas on the Action of Caustic 



till the last stage of solution lias been reached or has even 

 become far advanced. 



AjiJirocalUstes Bocagei^ Dactylocalyx pumiceus^ Farrea densa^ 

 Myliusia callocyathes. 



The sponges next submitted to experiment were the four 

 just named, of which A. Bocagei and D. ■pumiceus have been 

 found both fresh and in the deciduous state, and consequently 

 with fibres both channelled and solid, while the two latter, like 

 B. suhglobosa, are known only in the fresh condition with solid 

 glassy fibres. In all, however, results were obtained on boil- 

 ing with caustic potash precisely similar to those already 

 described in the case of B. suhglobosa. The canals of the im- 

 bedded spicules were enlarged and made visible ; the enlarge- 

 ment continued till the entire spicule was dissolved out ; and 

 finally the spicular casts ran together and the fibres became 

 hollow tubes : the rosettes and loose sexradiates soon passed 

 into solution ; while the spines of the fibre persisted for a con- 

 siderable time, traces of them remaining when the canals had 

 attained their fullest development. 



The attached sexradiates of many vitreo-hexactinellids, 

 when the siliceous cement has only just commenced to spread 

 over them, are frequently capitate at the extremities of their 

 rays, as though the enveloping silica had been fluid and accu- 

 mulated in greater quantity at the points of the rays than 

 along their sides, as, in fact, the sarcode from which the silica is 

 deposited may have done. Curiously enough, the casts of the 

 spicular rays excavated in the siliceous fibre frequently ter- 

 minate in the same capitate manner ; and it is possible that 

 when they do so the retrogressive metamorphosis of the fibre 

 has brought it back to exhibit in an inverse form the same 

 early stage as that to which we have just alluded. 



Euplectella aspergillum. 



The axial canals of both its free and combined spicules were 

 first developed ; and a concentric laminar structure was next 

 exposed both in them and in the cementing material, which 

 here only binds the spicules into bundles without absolutely 

 enveloping them. As in time the cementing silica becomes 

 wholly dissolved away in places, the spicules it unites together 

 are to a great extent set free ; and the composition of the lon- 

 gitudinal and transverse fibres formed by them is then made 

 manifest. 



Hyalonema Sieboldii. 



Mr. Higgin, of Liverpool, gave me a beautiful specimen of 



