566 Transactions of the Society. 



depressions, circular in outline and very shallow. This appears 

 to be the first stage of the process of penetration, which always 

 occurs from without. 



The third condition is the commonest, and is a combination of 

 the two others. Open axial canals and penetrations from without, 

 into the normal or enlarged canals are to be observed in the 

 majority of the spicula noticed in this communication. 



It is evident that these abnoriual states of the spicula tend to 

 diminish the quantity of silica in them, and are destructive in the 

 long run. The number of fragments of spicula in the deposit is 

 very remarkable, and most of them appear to have been produced by 

 gradual thinning of the spicule fi'om within outwards, and by 

 solution of continuity due to repeated penetrations. 



Fracture from thinning is shown in Figs, le, Id, 5c, %, and 

 10 ; and from the presence of perforations in Figs. 4:d, 10a, 5. 



Mr. H. J. Carter, F.E.S., noticed, in 1873, that spicules of sponges 

 were often penetrated from without, by holes leading to minute 

 canals, and that the axial canals, when they were open at the end, 

 became enlarged. He does not seem to have found the axial canal 

 penetrated by the minute canals from without, and no other than 

 simple enlargement of the axial part was noticed by him. He 

 wrote : * — " Taking the siliceous sponge spicule by itself we find 

 that it is subject to two kinds of wasting or decay, viz. one which 

 takes place in the interior or wall of the central canal, and the 

 other on the surface, the former frequently occurring in the living 

 sponge, and the latter in the substance of the spicule after death. 

 The wasting which takes place in the wall of the central canal is 

 recognized by its increasing size, which in some cases goes on until 

 the spicule is reduced to a mere shell, or it takes place only at the 

 ends of the spicule, where the central canal at these points presents 

 a funnel-shaped cavity diminishing inwards or towards the centre of 

 the spicule. In either case the cause is not apparent. As this 

 occurs in the living state, it is just possible that the central canal 

 of the spicule, which begins in a simple cell, may sometimes become 

 so dilated as to assume the form of a full-grown spicule with little, 

 if any, vitrification, and then appear as a mere shell." " On the 

 other hand, the destruction which takes place on the surface of the 

 spicule and extends into its substance, presents itself under three 

 difiierent phases, viz. first it consists of a simple superficial circular 

 cavity, which may increase in size and depth ; second, of a simple, 

 straight, uniform, bhnd tube extended vertically into the substance 

 of the spicule : and third, of a smaller tube of the same kind ending 

 in a globular dilatation. 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 

 * Ana. and Mag. Nat. Hist., xii. (1873) p. 456, plate xvi. 



