40 SPONGES 



distinction must be drawn between true or primary spicules which owe 

 their first origin to a single mother cell, and secondary spicules which 

 can be traced back to more than one cell Secondary spicules may 

 be due either to a deposit, not of spicular nature (see below, p. 41), 

 of skeletal material upon a primary spicule ; or to union of several 

 primary spicules to form a spicular system. The latter are usually 

 many -rayed forms, such as the equiangular triradiate and quadri- 

 radiate systems of many Calcarea (see below, p. 107), in which each 

 ray represents a distinct primary spicule or spicular element, derived 

 from its own mother cell or actinoblast, and fused secondarily with 

 its fellows to form the spicular system. The distinction between 

 these primary and secondary spicules is, however, one entirely in- 

 dependent of their morphological characteristics, since in Demo- 

 spongiae the spicules, with few exceptions, whatever their form or 

 the number of their rays, appear to arise from a single mother 

 cell ; while, on the other hand, many spicular systems in Cal- 

 carea have become secondarily monaxon in form. Nothing is 

 known with regard to the formation of the triaxon spicules of 

 Hexactinellids. 



The development of a primary spicule is very uniform, and that of 

 a simple monaxon type may be described in general terms as a typical 

 example (cf. Fig. 49, h-n). The first portion to be formed is a 

 minute organic rod, placed near the nucleus of the secreting cell. 

 This is the rudiment of the organic axial thread, and round it is 

 deposited the mineral matter. 



In calcareous spicules the organic axis is very slender, and 

 the mineral matter subsequently deposited is of a crystalline nature, 

 and almost, if not entirely, free from organic matter; the whole 

 spicule is enveloped in an organic sheath of the same nature as the 

 axial thread, and continuous with it at the apex of the spicule. 

 Sheath and thread are the oldest parts of the spicule, and probably 

 appear first as a minute cell vacuole in which a crystalline deposit 

 subsequently takes place to form the spicule round a denser central 

 portion which becomes the axial thread. The substance of the 

 vacuole, and consequently of the sheath and thread, is of the same 

 nature as the intercellular ground substance or mesogloea of the 

 dermal parenchyma. 



In siliceous spicules the organic axis is relatively much larger and 

 more conspicuous. The mineral matter is deposited round it in con- 

 centric lamellae of colloid silica, alternating with lamellae of organic 

 nature. One such organic coat probably forms an outer sheath to the 

 spicule, which is not, however, so conspicuous as in calcareous spicules. 

 The organic portions of the spicules grow faster than the mineral 

 portions, so that the axial thread projects at the two extremities of 

 the spicule rays into the protoplasm of the secreting cell. Hence 

 each spicule when freed from organic matter represents an open tube, 



