1. The first appearance of a calcareous spicule or spicular element, both ancestrally and in the actual development, was probably a minute vacuole in a cell of the dermal layer, filled with an organic substance perhaps identical with the intercellular ground substance, within which the minute sclerite appeared as a crystal or concretion.
2. The ancestral sclerite, though crystalline in structure, soon assumed a non-crystalline form as a whole, as an adaptation to its secondarily acquired function of support, and as it grew in size the contents of the vacuole formed the spicule sheath.
3. The ancestral form of spicule in the Calcarea was a simple monaxon, placed tangentially and completely embedded in the body-wall, lying between two adjacent pores.
4. From this ancestral spicule the forms of spicule now occurring in the Calcarea arose as follows: (a) the primitive monaxon acquired a distal portion projecting from the surface, as in the existing primary monaxons; (b) groups consisting each of three primitive monaxons became united by their contiguous ends to form a single triradiate system; (c) to some of the triradiate systems thus formed a fourth ray was added, secreted by the pore-cell, giving rise to the quadriradiate system ; (d) some of the triradiate systems, by loss of one ray and placing of the other two in a straight line, or by loss of two rays, perhaps became modified into secondary monaxon spicules.
5. The power of secreting a monaxon sclerite was primitively possessed by every cell of the dermal layer, and this condition appears to be retained in Leucosolenia. In Clathrina, on the other hand, all the skeletogenous cells migrate inwards from the dermal epithelium, and form a connective-tissue layer distinct in function from the contractile, undifferentiated dermal epithelium. In Leucosolenia also the actinoblasts of the triradiate systems form a deeper layer, but the dermal epithelium secretes primary monaxons--at least in the young form--and is non-contractile.
6. The forms of the spicules are the result of adaptation to the requirements of the sponge as a whole, produced by the action of natural selection upon variation in every direction.
Crepant resolutions and open strings II
