IX] OF INTRACELLULAK SPICULES 443 



the same or very similar phenomena to occur in other organisms, 

 wherever the conditions of inorganic secretion within closed cells 

 was very much the same. As a matter of fact, in the group of 

 Holothuroidea, where the formation of intracellular spicules is a 

 characteristic feature of the group, all the principal types of 

 conformation which we have just described can be closely 

 paralleled. Indeed in many cases, the forms of the Holothurian 

 spicules are identical and indistinguishable from those of the 

 sponges*. But the Holothurian spicules are composed of calcium 

 carbonate while those which we have just described in the case 

 of sponges are usually, if not always, siliceous : this being just 

 another proof of the fact that in such cases the form of the 

 spicule is not due to its chemical nature or molecular structure, 

 but to the external forces to which, during its growth, the 

 spicule is submitted. 



So much for that comparatively limited class of sponge- 

 spicules whose forms seem capable of explanation on the hypothesis 

 that they are developed within, or under the restraint imposed by, 

 the surface of a cell or vesicle. Such spicules are usually of small 

 size, as well as of comparatively simple form ; and they are greatly 

 outstripped in number, in size, and in supposed importance as 

 guides to zoological classification, by another class of spicules. 

 This new class includes such as we have supposed to be capable 

 of explanation on the assumption that they develop in association 

 (of some sort or another) with the lines of junction of contiguous 

 cells. They include the triradiate spicules of the calcareous 

 sponges, the quadriradiate or " tetractinellid " spicules which occur 

 in the same group, but more characteristically in certain siliceous 

 sponges known as the Tetractinellidae, and lastly perhaps (though 

 these last are admittedly somewhat harder to understand) the 

 six-rayed spicules of the Hexactinellids. 



The spicules of the calcareous sponges are commonly tri- 

 radiate, and the three radii are usually inchned to one another 

 at equal, or nearly equal angles ; in certain cases, two of the 

 three rays are nearly in a straight line, and at right angles to the 



* See for instance the plates in Theel's Monograph of the Challenger Holo- 

 thuroidea; also Sollas's Tetractinellida, p. Ixi. 



