420 OUTLINES OF EVOLUTIONARY BIOLOGY 



the teeth of these microscopic and apparently useless organs 

 useless at any rate so far as their generic and specific characters 

 are concerned, for what can it matter to the sponge whether the 

 number of the teeth be three or more or less, or whether the teeth 

 at the two ends of the spicule be equal or unequal ? Yet, in the 

 course of evolution, such characters as these have become more 



FIG. 187. Siliceous Spicules (Chelse) of Sponges. (After Ridley and 

 Dendy, in " Challenger " Report.) 



A. A', front and side views of chela of Esperella lapidiformis, x 360. 



B. B', front and side views of chela of Esperiopsis pulchella, x 284. 



C. C', front and side views of chela of Cladorhiza (?) tridentata, x 360. 



or less fixed and constant and they are evidently handed down 

 from generation to generation by the ordinary process of heredity. 1 

 We pointed out in an earlier chapter that the external form 

 of the entire sponge is, in some cases at any rate, explicable 

 as an adaptation to peculiar conditions of the environment. 

 We saw this is the case of the curious " Crinorhiza " form 

 (Fig. 168) which prevents the sponge from sinking into the soft 

 mud or ooze on which it rests. Let us glance for a moment, 

 however, at another deep sea sponge, Esperiopsis challengeri, 

 dredged up by the "Challenger" Expedition from a depth of 



1 The reader should refer back to Fig. 88 for other forms 01 sponge spicules. 



