to be conformed in direct relation to the forms and the arrangement of the cells or other protoplasmic elements49. {436} 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 lines of Hartingãs and Raineyãs experiments, that is to say as the direct result of chemical or physical phenomena associated with the deposition of lime or of silica in presence of colloids50. This is the case, for instance, with various small spicules of a globular or spheroidal form, formed of amorphous silica, concentrically striated within, and often developing irregular knobs or tiny tubercles over their surfaces. In the aberrant sponge Astrosclera51, we have, to begin with, rounded, striated discs or globules, which in like manner are nothing more or less than the {437} ãcalôÙcoôÙspheôÙritesã of Hartingãs experiments; and as these grow they become closely aggregated together (Fig. 210), and assume an angular, polyhedral form, once more in complete accordance with the results of experiment52. 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 some