SPONGES 41 



with a minute lumen, the axial canal, formerly occupied by the 

 organic axis. 



Although a true spicule arises as an intracellular deposit, it 

 usually greatly outgrows the mother cell, and may attain relatively 

 gigantic proportions, as, for instance, in the spicules of the root tuft 

 of EiiplecteUa and Hijalonema. In such cases it is far from certain 

 how the later growth is eftected. It is commonly assumed that 

 other scleroblasts attach themselves to the growing si)icule and 

 deposit fresh mineral substance upon it. Growth of this kind 

 has, however, only been demonstrated in the case of the irregular 

 spicules known as desmas (see below, p. 134) of the Lithistida, spicules 

 clearly of a secondary nature. In Calcarea, on the other hand, the 

 whole growth of the spicule or spicular element is entirely due to 

 the activity of the original scleroblast and its descendants. The 

 mother cell divides into a greater or less number of formative 

 cells which spread over the growing spicule and build it up to the 

 required size. In other cases only the nucleus of the scleroblast 

 divides, and the spicule ray is enveloped in a nucleated plasmodium. 

 The later development of the spicules of Demospongiac has not 

 been studied, but it is probable that, as in Calcarea, all true 

 spicules, Avhatever their size, are secreted entirely by the mother 

 cell or by cells derived from it. 



When the spicule is fully formed the scleroblast, or at least 

 some of the formative cells derived from it, may persist, adhering 

 to the spicule after their secretive activity has ceased, as is always 

 the case in Calcarea ; or they may disappear from the spicule when 

 its growth is complete, as seems always to occur in the case of 

 siliceous spicules. 



The above account of spicule development applies equally to the 

 individual raj^s of the secondary spicular systems in Calcarea, an 

 account of which Avill be found below (p. 107). 



In addition to the secretion of mineral substance in the form of 

 spicules, secondary deposits of silica are formed on the desmas, 

 already mentioned of Lithistida, and in the form of cement, uniting 

 spicules together, in Hexactinellids. It is not known accurately in 

 any case how these deposits are laid down, but it is very possible 

 from the mode of their formation that they represent secretions of 

 a cuticular or extracellular nature, and are therefore very ditt'erent 

 from the spicules. 



A true spicule may, in short, be defined as an intracellular 

 secretion of skeletal material, formed either by a single mother cell, 

 or by a number of formative cells all derived from one such mother 

 cell. 



(3. Spomjin is an organic substance allied to silk, but apparently 

 of variable composition. It is generally stated to yield leucin and 

 glycin, but not tyrosin, when heated with sulphuric acid, and its 



