ECHINODERMA GENERAL DESCRIPTION 29 



reticulum becomes a more definite stroma, so the pattern of the 

 stereom acquires definiteness, and varies in the different parts of 

 an individual, as well as in different species. In the course of 

 ages the spicules of the adult have themselves come to acquire 

 definite shapes characteristic of species, and this is notably the 

 case with the complicated " wheels," " anchors," and " tables " of 

 Holothurians (Figs. II. 3, 6; V. 6, 7, pp. 222, 233) ; but, as Stewart 

 and Bell have shown, also applies to the spicules of the thecal 

 cavity in sea-urchins. The spicules of the theca usually fuse into 

 plates, those of the appendages (brachia, stem, etc.) into ossicles. 

 There is no real distinction ; but it is often the case that the 

 reticulum of the ossicles runs in straighter lines, while that of the 

 plates is a more open mesh-work ; this is due to the definite 

 arrangement of the connecting fibres of the stroma in the append- 

 ages, and when this is definite in the theca the result, as we shall 

 see under Cystidea (p. 42), is the same. Attempts to use this 

 as an important character in distinguishing brachials from radials 

 (p. 112), or a dorso- central from columnals, have no secure 

 foundation. The stereom is absorbed by cells similar in outward 

 appearance to those which deposit it, and the calcareous salts are 

 transmitted by communicating pseudopodia from the absorbent to 

 fresh depositing cells. Thus the spicules of the Echinoid Plvteus 

 form a reserve for the growing urchin ; thus, too, the anal plate 

 of the growing Antedon is absorbed, and its material used by the 

 increasing radials. Theel, to whose observations (1892-96) much 

 of this knowledge is due, compares the reticular tissue, the 

 osteoblasts, the osteoclasts, and the " Howship's foveolae " formed 

 by the latter, of Vertebrata with the similar structures in Echino- 

 derma. Bone in its first stages, especially that formed in connec- 

 tive tissue, is marvellously like Echinoderm stereom, and is like- 

 wise of mesodermic origin. But, whereas bone is an extracellular 

 formation, it is probable that the spicules of Echinoderms, like 

 those of Sponges, are intracellular. The otoliths of some 

 Holothurians and the biscuit spicules of others (see p. 224) are 

 distinctly intracellular formations. Bone, moreover, is phos- 

 phate, not carbonate, of lime, and does not retain the markedly 

 crystalline character always possessed by Echinoderm stereom, 

 even when highly complicated. Each skeletal element of an 

 Echinoderm acts as a crystallographic individual, polarising light 

 and cleaving along the planes characteristic of calcite. In fossils 

 the cleavage is often emphasised by an infilling of the spaces with 

 secondary calcite which has axes identical with those of the original 

 crystal. According to Semon (1887), every skeletal element begins 

 as a tetrahedron, usually in the form of a trifid spine with branches at 

 an angle of 120. The formation of similar spicules in sponges has 

 been studied by Minchin (1898, see Part II., Chapter on Porifera). 



