SoLLAS — On Vetulina stalactites {0. S.), 8^c. 489 



Similarly the granular axes of the rays are more readily dissolved than 

 the periphery. 



It appears probable from these observations that a cell is asso- 

 ciated with the young spicule, and that the nucleus with its nucleolus, 

 both large like the corresponding structures in a Geodia globule, be- 

 come inclosed in the substance of the centrum, as this continues its 

 growth ; subsequently silica is irregularly deposited within the nu- 

 cleus, which, decaying away, leaves a number of empty spaces then 

 recognizable as air cavities or granules. So, too, in the axis of the 

 rays, a substance like that which forms the axis of a spicule is 

 deposited, but in a more irregular manner, and this, disappearing 

 after a time, leaves the granulations, or air-spaces, to represent it. 



Schmidt could not find the oval granular space in young cor- 

 puscles, and as this seemed to me to require explanation, I wished to 

 obtain material for investigation. Having used up the small morsel 

 of Vetulina I had obtained when Curator of the Bristol ]!L[useum, I 

 applied to the Honorary Secretary, llr. S. H. Swayne, expressing a 

 desire to obtain a further supply, and I now take this opportunity of 

 acknowledging the ready kindness with which he responded to my 

 wishes. I boiled the specimen thus obtained in nitric acid, and as it 

 presented a large unbroken surface, I expected to obtain a large crop 

 of young corpuscles : the result, however, was disappointing — not 

 more than half a dozen young forms were secured. These, however, 

 differ markedly from the adult forms, and no less from the represen- 

 tation which Schmidt gives of his youngest stages. The youngest 

 form found is shown in fig. 2, plate rv. Like all the other early 

 forms, it consists of a flat plate, from which a number of arms radiate ; 

 both arms and disc possess the corroded appearance so characteristic of 

 young Lithistid corpuscles. In the centre is a clear round space, and 

 within this an excentric round body. In other examples I found the 

 flat body of the corpuscle completely perforated by an oval space, 

 showing that the silicification which had produced the rest of the 

 corpuscle had been arrested in the centre over a definitely limited 

 area. As this is precisely similar to what takes place in a Greodia 

 globule, the hilum there representing the space occupied by the 

 nucleus of the spicule-cell, it seems fair to conclude that a nucleus 

 does exist in connexion with the young corpuscles of Yetulina, but 

 subsequently disappears, as it does in the Geodia globule ; only in the 

 case of Vetulina the cavity representing the hilum becomes sub- 

 sequently completely enclosed within the body of the corpuscle. 



Having satisfied ourselves as to the true form of the corpuscles of 

 Vetulina, we may next inquire as to the manner in which they are 

 united together — for this, a special method of examination now to be 

 explained, is necessary. In the first place it is to be observed that 

 when the opal of which a Lithistid corpuscle chiefly consists is dis- 

 solved by caustic potash there still remains a residue insoluble 

 in potash. Of its chemical nature I am at present ignorant. It 

 remains as a very tender, delicate film, readily soluble in dilute 



