68 Mr. G. P. Bidder. 



the existing triangular grouping 1 of the formative cells determined the 

 crystallisation of the calcite in a flattened (pinacoid) form, extended 

 transversely to the optic axis, as in Schiefer spar : the unicellular 

 crystals appear as [elongated] rhombohedra [c/. Sollas (9) p. 389] 

 which form the primitive acerates. The ontogenetic history of 

 the triradiates of Leucosolenia is not yet known. It is possible that 

 the rudiments originally form an acute angle with the morpho- 

 logical axis of the sponge, and that the formative cells repeat a 

 change which has taken place in evolution, with the advantage 

 of the production of a [gastral from an ectocytal] skeleton. The 

 causes governing the direction of the optic axis in this and higher 

 sponges are still not clear.* 



Whatever the cause, the u alate " spicule (fig. 9), such as is 

 typical of Leucosolenia, and very frequent in more complex sponges, 

 is stated by Sollas (9) and Ebner to have its optic axis at an acute 

 angle with the morphological axis of the unpaired ray, in a plane 



* [The acicular spicules which clothe S. raphanus appear completely referable 

 to law. In the body of the radial tubes the optic axis of the triradiates is nearly 

 parallel with the axis of the tube ; this we may ascribe to compression, radial to the 

 sponge, due to tension of the cylindrical outer surface. On the free conical ends 

 of the tubes the optic axis is nearly perpendicular to the surface ; the large acicular 

 spicules leave the surface tangentially, and can frequently be seen to be the extremely 

 prolonged unpaired rays of triradiates (of. Ebner and others) ; corresponding with 

 these relations the optic axis makes an angle of nearly or exactly 90 with the 

 morphological axis. The fine Acicular spicules leave the surface nearly vertically, 

 and the optic axis is correspondingly coincident with their length ; like the gastral 

 rays of quadriradiates in Ascaltis they are to be regarded crystallographically as 

 extremely acute rhombohedra. This is the acicular form which we should expect 

 to find in free crystallisation of calcite, and I have shown elsewhere (19) though 

 suggesting a teleological explanation that the number and size of these fine 

 spicules vary greatly with the condition of the water. 



In L. Lieberkuhnii the optic axis is often exactly at right angles to the acicular 

 spicules, in other cases it makes with the long axis an angle of from 70 upwards. 

 The figures of Schulze (S. raphanus) and Minchin (L. variabilis) suggest that the 

 larval spicules are tangential when first formed, and that some are afterwards 

 rotated outwards. It does not seem at present possible to account for the varying 

 angles made by the optic axis to the morphological axis of acicular spicules, with- 

 out reference to cellular directive power. The giant oxeotes of the young Leu- 

 candra aspera especially suggest that there has been cellular determination to form 

 a divergent brush of protective spicules. To examine thij question there is 

 necessary greater knowledge as to the mode of ontogenetic development of the 

 canal-system, as to the mechanical strains resulting in the various stages of such 

 development, and as to the laws governing free crystallisation of calcite in acicular 

 forms of which the greatest length is not parallel to the optic axis. 



In the club-spicules of & compressum the optic axis lies in the plane of the 

 spicule, normal to the morphological axis at its point of greatest curvature. It 

 must be noticed that, according to the observation recorded above, the acicular 

 spicules of the adult sponge are in this species not unicellular in formation. July 

 23, 1898.] 



