of the Spicule of sponges. 431 



these figures are drawn to the same scale, viz. l-24th to 

 l-600th of an inch, whereby they furnish facsimiles equally 

 magnified of the objects they are intended to represent. 

 Fig. 3 is the head of an average anchor-spicule taken from 

 the base of the adult form (fig. 1) and magnified to the same 

 scale. It not only also shows an extension of the central canal 

 beyond the branches given off" for the arms, but points out the 

 relative sizes of the adult and foetal spicules of this kind, when 

 compared with d of the following figure. 



There are other spicules in the young Tethya, especially 

 fork-heads having one, two, or three arms ; and these are re- 

 presented under fig, 17 : a is one-armed, analogous to the one- 

 armed anchor-head just described ; c has two arms, and is the 

 most numerous form in the young Tethya after the one-armed 

 anchor ; b is the three-armed form, which is scantily present, 

 like the three-armed anchor-head, d\ while e is the acerate 

 form. I need hardly add that the other ends of all these spi- 

 cules are single-pointed. 



Thus the development of the arm is always accompanied by 

 an extension of the central canal of the shaft. But there are 

 other additions to the spicule, viz. spines &c., which are not 

 always so accompanied, as may be seen by reference to fig. 4, 

 PI. XXII., where they may be observed to have been added 

 to the outside of the shaft after the latter had been formed. 

 Hence, in this instance, it is not the extension of the central 

 canal which determines the ultimate form of the spicule, but 

 some external agency, which adds to and modifies the external 

 form of both the horny and silicified fibre, as well as the spi- 

 cules. That this should be easily effected on all sides in the 

 midst of the sponge, where these parts are enveloped in the 

 intercellular sarcode, may be easily conceived 5 but it is not 

 easy to conceive how this takes place in the long spicules of 

 Hyalonema and Holtenia^ unless they grow, like hairs, by ad- 

 ditions to their proximal extremities, or the sarcode creeps out 

 over them to their ultimate terminations. 



Still, we are dealing here Avith the developments of the spi- 

 cule after the shaft has been formed, and not with its earliest 

 appearance, to which I can add nothing more than I stated in 

 1849, I. c, viz. " My impression still is, that both the horny 

 skeleton and its spicules are formed in the intercellular sub- 

 stance, and not in the cells." But how they come into being 

 I know not, any more than the " Preacher," who, 3000 years 

 ago, wrote : — 



" As thou knowest not what is the way of the spirit, nor 

 how the bones do grow in the womb of her that is with child ; 



