OF THE SPONGIAD^. 13 



claviger, which appear to be anabgous to the rectangulated- 

 triradiate spicula of Grantia; and they also state that in 

 the first-named species crucial or dagger-shaped spicula 

 occur in the branchiae and margins of the cloak of the 

 animal, and forms very similar to those occur on the inter- 

 stitial membrane oiLeuconia nivea, Bowerbank. Numerous 

 forms of tuberculated and smooth calcareous spicula are also 

 found in the extensive family of the Gorgoniadse. And the 

 siliceous simple bihamate form of retentive spiculum, so 

 abundant on the interstitial membranes of many species of 

 sponges, are closely represented by the calcareous bihamate 

 spicula so numerous on the tubular suckers of Echinus 

 sphcera. Thus we find in the spicula only, a series of finks 

 in the chain of animal development, intimately connecting 

 the Spongiadse with the higher tribes of animals. 



In the soHd sfiiceous fibres of Bactylocalyx (Pig. 374, 

 Plate XV), and in the tubular siliceous fibres oiFarrea occa, 

 Bowerbank, MS. (Fig. 277, Plate XV), and especially in 

 the latter, we obtain a very much closer approximation to the 

 tubular forms of the bones of the higher classes of animals. 



Prom our knowledge of the great scheme of the natural 

 development of animal life, the most perfectly organized 

 sponges appear to be those which secrete carbonate of lime 

 as the earthly basis of their skeletons^ and the least perfect 

 those which secrete no earthy matter in the skeletons; 

 those which secrete silex taking an intermediate position ; 

 but it must also be remembered that there is no form of 

 spiculum found among the calcareous sponges, or in the 

 higher tribes of animal fife, that is not repeated among the 

 siliceous forms of spicula of the Spongiadse. 



The spicula may be conveniently classed under the fol- 

 lowing heads : 



1. The essential skeleton spicula. 

 3. The auxiliary spicula. 



The Essential Skeleton Spicula. 



In the siliceous sponges they are usually simple, elongate 

 in form, slightly curved, and occasionally more or less fur- 



