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



spicule yields to the action of the potash and the fine point is 

 quickly dissolved away, the axial canal is soon exposed and 

 the potash finds easy access to the interior of the spicule. 

 The axial canal then rapidly begins to enlarge, and soon ex- 

 pands into a funnel-shaped termination towards its extremities, 

 or where the potash first enters. The sides of this funnel- 

 shaped expansion, when viewed with a high power, do not 

 appear in optical section as simple straight lines, but are very 

 finely " stepped " all the way down, each step representing 

 very obviously the exposed edge of one of the concentric la- 

 mellse of which the spicule is made up. The concentrically 

 lamellar structure of these and all other true spicules is, indeed, 

 beautifully revealed by this method of solution, more perfectly 

 and in a far clearer way than by the process of charring the 

 spicule with hear, as in the flame of a spirit-lamp. 



As solution proceeds, the points of the spicules become en- 

 tirely removed, and a fusiform acerate spicule becomes almost 

 cylindrical from the loss of its conical ends : but the diflterent 

 lamellae never dissolve at the same rate; some always resist 

 longer than others ; so that at every stage of solution the spi- 

 cule presents a lamellar appearance. The outermost lamella 

 frequently endures the longest, forming a hollow sheath of 

 infinite tenuity, which frequently splits longitudinally so as to 

 form two or more long narrow strips or ribbons ; and these, in 

 some instances, curl backwards and outwards in a very singu- 

 lar way. A fossil spicule in this last state would not be readily 

 recognized for what it actually was, and has been, indeed, on 

 more than one occasion a source of considerable difiiculty to 

 myself in studying the spicules of the Cambridge coprolites. 



The anchoring spicules undergo just the same changes as 

 the acerates; but their short conical arms dissolve much faster 

 interiorly than the longer shafts, because the edges of a greater 

 number of concentric lamellee are exposed over a shorter length 

 than in the shaft, and so present in the same space a larger 

 number of ^jomfe cVapj^ui, so to speak, for the action of the 

 caustic potash. 



The spicules which undergo the most remarkable alteration, 

 however, are the " globo-stellates," which Bowerbank mistook 

 for ovaria. Tiie interior of these, at first optically homoge- 

 neous, "soon exhibits a radiate structure starting from the centre 

 (PI. IX. fig. 6), and with its separate rays terminating at un- 

 equal distances short of the exterior ; as solution continues a 

 small cavity is developed in the centre, the radiate structure 

 is better displayed and extends much further towards the cir- 

 cumference (PL IX. figs. 9, 8), the small cavity continuously 

 enlarges till at length it occupies the whole interior of the 



