IX] AND SPICULAR SKELETONS 671 



forces, we shall presently pass to that other and larger group which 

 appears to be conformed in direct relation to the forms and the 

 arrangement of cells or other protoplasmic elements*. The two 

 principles of conformation are both illustrated in the spicular 

 skeletons of the sponges. 



In a considerable number but withal a minority of cases, the 

 form of the sponge-spicule may be deemed sufficiently explained 

 on the Hnes of Harting's and Rainey's experiments, that is to say 

 as the direct result of chemical or physical phenomena associated 

 with the deposition of Ume or of silica in presence of colloids f. 

 This is the case, for instance, with various small spicules of a 

 globular or spheroidal form, consisting of amorphous silica, con- 

 centrically striated within, and often developing irregular knobs 

 or tiny tubercles over their surfaces. In the aberrant sponge 

 AstroscleraX, we have, to begin with, rounded, striated discs or 

 globules, which in hke manner are nothing more nor less than the 

 calcospherites of Harting's experiments; and as these grow 

 they become closely aggregated together (Fig. 309), and assume an 

 angular, polyhedral form, once more in complete accordance with 

 the results of experiment §. Again, in many monaxonid sponges, 

 we have irregularly shaped, or branched spicules, roughened or 

 tuberculated by secondary superficial deposits, and reminding one 

 of the spicules of the Alcyonaria. These also must be looked 

 upon as the simple result of chemical deposition, the form of the 

 deposit being somewhat modified in conformity with the surrounding 

 tissues: just as in the simple experiment the form of the con- 

 cretionary precipitate is affected by the heterogeneity, visible or 

 invisible, of the matrix. Lastly, the simple needles of amorphous 



* Cf. Fr. Dreyer, Die Principien der Geriistbildung bei Rhizopoden, Spongien 

 und Echinodermen, Jen. Zeitschr. xxvi, pp. 204-468, 1892. 



t In a very anomalous Australian sponge, described by Professor Dendy {Nature, 

 May 18, 1916, p. 253) under the name of Collosderophora, the spicules are 

 "gelatinous," consisting of a gel of colloid silica with a high percentage of water. 

 It is not stated whether an organic colloid is present together with the silica. 

 These gelatinous spicules arise as exudations on the outer surface of cells, and 

 come to lie in intercellular spaces or vesicles. 



J J. J. Lister, in Willey's Zoological Results, pt iv, p. 459, 1900. 



§ The peculiar spicules of Astrosdera are said to consist of spherules, or calco- 

 spherites, of aragonite, spores of a certain red seaweed forming ^ the nuclei or 

 starting-points of the concretions (R. Kirkpatrick, Proc. E.S. (B), lxxxiv, p. 579, 

 1911). 



