The nature and minute structure of calcareous spicules, studied in the large monaxons of Leucandra aspera. The spicules show a very distinct finely alveolar structure when heated, but do not at the same time increase in thickness, as do siliceous spicules under similar conditions, so that the resulting distinctness of the alveolar structure cannot be attributed to the enlargement of spaces originally present, but possibly to a fusion of minute cavities into larger, visible alveoli. The spicules show no trace of an axial thread. The so-called sheath which remains when the spicules are dissolved in strong potash appears not to consist purely of organic matter, but is composed also of inorganic material in some form, possibly CaCO3; BUTSCHLI (9). .The nature and minute structure of siliceous spicules, studied in the macroscleres of Geodia placenta and Tethya lyncurium. The siliceous substance is amorphous silica, corresponding to mineral opal. Heating the spicules produces a finely alveolar system of minute closed spaces, probably present in the normal unaltered condition, but too fine to be seen until the alveoli have been enlarged by the action of heat in converting into vapour the water contained in them. The stratified structure of the spicule is further evidence for this view. The axial thread shows the reactions characteristic of albuminous substances (pp. 262-267), and in the normal condition is firm and brittle, but becomes softened when isolated by hydrofluoric acid. A certain amount of organic matter is contained also in the siliceous substance of the spicule. The spicules of Tethya are enveloped by a great number of richly branched and anastomosing bands, which stain deeply with dahlia, and evidently represent the silicoblasts (p. 269, text figg. a & b), [cf. MAAS (37) II, B, iv, c infra]; and in spicules in which the axial thread is deeply stained and shrunk, there can be seen not infrequently bodies similar to cells, generally much branched, at regular intervals between the axial thread and the wall of the axial canal (p. 271, pl. xxi, figg. 10-12); BUTSCHLI (9). .Other formative cells become opposed to the original scleroblast, so that the large monaxons are covered by cells, arranged either as an epithelium or as a syncytial layer. When the spicule is complete the formative cells appear to break up and perish. [Cf. BUTSCHLI (9), supra, II, B, iv, b.]