i8 95 . NOTES AND COMMENTS. 77 



the lecturer touched on many interesting points regarding the origin 

 and growth of oolitic granules, full consideration of these latter 

 points, however, being deferred to some future occasion. We shall 

 be much interested to see this paper in print. 



The Formation and Absorption of Skeletal Substance in 

 echinoderms. 



In his valuable memoir "On the development of Echinocyamus 

 pusillus" (Nov. Act. Reg. Soc. Sci. Upsala, ser. iii., 1892), Professor 

 Hjalmar Theel showed that the calcareous spicules were deposited 

 within a protoplasmic mass formed by the fused pseudopodia of 

 several wandering, amoeboid cells. The skeletal substance, or 

 stereom, has therefore an intracellular origin. In a short note 

 recently communicated to the Royal Academy of Science at Stock- 

 holm (Ofuersigt, 1L, p. 345), Professor Theel confirms his former 

 views, and shows that similar amoeboid cells have also the power of 

 absorbing the stereom ; so that the two groups of cells correspond to 

 the osteoblasts and osteoclasts of the vertebrates. The absorbent 

 cells only differ from the formative cells in the greater activity of 

 their amoeboid movements, which give the impression that the 

 process of absorption demands intenser labour on the part of the 

 cell than does that of deposition. When a cell begins to absorb a 

 spicule it strains to extend and flow round and over it, so as to take it 

 whole into its protoplasm ; hence the granular main portion of the 

 cell moves incessantly, gliding slowly along the swallowed spicule 

 until nothing remains of it. Meanwhile the pseudopodia are con- 

 tinuously extended and retracted. Since the calcareous particle 

 attacked is often of considerable size as compared with the amoeboid 

 cell, we have to suppose either that the latter can dissolve an 

 unexpectedly large quantity of salts and retain them in solution, or 

 that, as is more probable, the dissolved salts are gradually transferred 

 through the pseudopodia to other cells in the neighbourhood, which 

 may either retain them till needed, or use them immediately for 

 building up fresh calcareous structures in the growing Echinoderm. 

 When the calcareous body is too large to be completely enveloped by 

 the absorbent cell a portion of it is detached, apparently by the 

 secretion of an acid, and this portion is then swallowed. A piece of 

 stereom, so large that it can hardly be taken into the protoplasm of the 

 cell, is fully dissolved in about two hours. 



This process of absorption not improbably takes place, pari passu 

 with the processes of growth, all through the life of an Echinoderm. 

 A familiar instance of absorption is seen in the orals of many crinoids. 

 The process is best observed, however, at the close of the larval 

 period, when the skeleton of the larva is giving place to that of the 

 adult form. In the pluteus or sea-urchin larva, for instance, at this 

 period not only is the larval skeleton in the way, but a large number 



