OF THE SPONGIAD^. 61 



linear power of 660, in the shape of slender, smooth, 

 cylindrical spicula, with a slight enlargement at each ter- 

 mination, and without the slightest indication of spines on 

 the shaft ; and in this condition the central cavity is large, 

 occupying about one third of its diameter (Pig. 310, 

 Plate IX). In the second stage, the only alteration in its 

 form is an enlargement of the terminations, the edges 

 assuming an angular shape, and a few slender spines are 

 observable (Fig. 311). In the third stage of development 

 the terminations assume the form of distinct circular plates 

 or incipient rotulse, the margins of which are slightly 

 crenate ; the shaft exhibits numerous long slender spines, 

 and the central cavity now does not occupy more than one 

 fifth of the diameter of the spiculum (Fig. 313). From 

 this form specimens in every stage of development may be 

 readily traced, until the strongly spinous margin, the pro- 

 minent convexity of the rotulae, and the robust shaft with 

 its long conical spines, indicate the completely adult con- 

 dition of the spiculum, and in this state the central cavity 

 can very rarely be seen (Fig. 308). 



The growth of these spicula in their early stages is 

 probably very rapid, as the number of those in the first 

 and second stages is remarkably small as compared with 

 those in the third and subsequent stages. 



In the inequi-birotulate spicula of Spongilla paulula, 

 Bowerbank, we find a number of radial canals passing 

 from each end of the central cavity of the shaft to the 

 extreme circumference of the rotulae ; and it is therefore 

 probable that this expanded part of the spiculum is similar 

 in character to that of the foUato-peltate spiculum which I 

 have described (Terminology, 103) in treating of the 

 spicula of the membranes ; and that they are, in fact, 

 originally composed of a series of terminal radial spicula 

 expanding and coalescing laterally, and thus forming one 

 plane circular surface in place of numerous separate 



radii (Fig. 333). u ■ .u ■ i 



The spicula of the third group, those havmg the spicula 

 disposed in fasciculi in the substance of the gemmule, difi'er 

 less in character from those of the parent sponge than 



