294 Mr. W. J. Sollas oyi the Action of Caustic 



have been sealed up ; at all events no internal solution had 

 taken place. 



The small stellates and other delicate spicules of this Geodia 

 dissolve and disappear at a very early stage, so that none of 

 them are to be seen in my slides amongst the partially dis- 

 solved larger spicules. 



Pacliymatisma Johnstonia. 



In the cylindrical spicule of this sponge the ends of the axial 

 canal are much further removed from the extremities, and con- 

 sequently better protected than they are in the case of the 

 acerates we noticed in Geodia] but since neither end of the 

 canal is better protected than the other, we find as a rule that 

 both are exposed at about the same time, and so in the majority 

 of these spicules the action of potash is to remove both ends and 

 develop the axial canal from each extremity, in just the same 

 way as in the case of the acerates of Geodia (PL IX. figs. 

 16, 17). 



I may here take the opportunity to record a very abnormal 

 occurrence in connexion with these spicules, that, viz., of two 

 of them existing in a state of comj^lete ankylosis. The axial 

 canals in this ankylosed structure have been made very visible 

 by the caustic potash, and can be seen crossing each other at 

 one point without intercommunication, so that each spicule 

 possesses its own distinct canal quite separate from that of its 

 fellow, and neither can be regarded as a mere process of the 

 other. The fact of their ankylosis is easily determined by 

 mere inspection (PI. IX. fig. 18). Bowerbank's dictum, that 

 spicules never ankylose together, not even by morbid growth, 

 is thus seen not to be of universal application*. 



The globo-stellates behave very much as those of Geodia'^ 

 but in the examples I have had before me one or two additional 

 phenomena have been observed.' In the first place the hemi- 

 spherical pits which are so common on fossil spicules are here 

 produced artificially by the action of the caustic potash in great 

 abundance, marking the spicules all over with regular excava- 

 tions of just the same size and appearance as those Avhich are 

 seen in the fossil examples (PI. IX. figs. 10, 11). The same 

 hemispherical pits also ap])ear on the acerate and large an- 

 choring spicules of Geodia arahica after boiling with potash ; 

 in these cases the successive lamiuas of the spicule are exposed 

 in circles concentric with tlie circumference of the excavation 

 (PI. IX. fig. 15). 



Next, in one or two instances a thin structureless exterior 



* See also 0. Schmidt, Spong. d. Kiiste v. Algier, p. 16, Taf. iii. fig. 2. 



