Hexactinellidse and Lithistidce. 457 



the keratose sponges to such an extent as often to render it 

 very difficult to determine what was its original form. 



On the other hand, the destruction which takes place on 

 the surface of the spicule and extends into its substance 

 presents itself under three different phases (PI. XVI. fig. 8) : 

 viz., first, it consists of a simple superficial circular concavity, 

 which may increase in size and depth (fig. 8, a) ; second, of a 

 simple, straight, uniform blind tube extended vertically into 

 the substance of the spicule (fig. 8, h) ; and, third, of a smaller 

 tube of the same kind ending in a globular dilatation (fig. 8, c). 

 In each instance it seems to be produced by the eroding action 

 of an organized cell ; that is, in specimens of the two latter, 

 mounted in Canada balsam, a granuliferous cell may be ob- 

 served to occupy the inner extremities respectively (fig. 9, a, 

 &, c), recalling strongly to mind the appearance of the sapro- 

 legneous cell Pythium when working its way through the cell- 

 wall of Spirogyra. Kolliker gives good figures of the first 

 and second forms of this, merely observing that it is a "peculiar 

 degeneration " (' Icones Histologicae,' der feinere Bau, p. 83, 

 pi. viii. fig. 10). 



It is the dimpled superficial kind of this destruction which, 

 attacking the deciduous spicule, seems not only to destroy the 

 ornamental parts but in many instances to reduce the spicule 

 to a mere ragged stick-like state, in which its original form 

 is no longer recognizable : hence the condition of a great 

 number of the fossil spicules in the Upper Greensand deposit 

 of Haldon Hill near Exeter (Annals, 1871, vol. vii. p. 113, 

 pis. vii., viii., & ix.). 



I may here also notice that the calcareous sponge-spicules 

 are also subject to two kinds of destruction, viz. : — one which 

 takes place in the living sponge, where the extremities of the 

 acerate long spicules are rendered funnel-shaped, as before 

 mentioned in the siliceous ones ; and thf other, in which there 

 is a general breakdown of the whole fabric, which gradually 

 becomes resolved into a group of aqueous-looking globules of 

 different sizes, among which there is not a trace of the original 

 structure to be seen. Were this change confined to those cal- 

 careous spicules which I have mounted in Canada balsam, I 

 should have inferred that it was caused by the balsam ; but I 

 find that the same change accompanies these spicules where 

 they may have been taken in by the kerataceous sponges to 

 form an axis for their horny fibre ; and it is worthy of remark 

 that the spicules of the Echinodcrmata, which may lie side by 

 side with them, do not appear to be similarly affected. Of 

 what nature the origin of this disorganization may be I am 

 ignorant ; it is a chemical question ; but the destruction takes 



