2 I 8 PORIFERA 



Tetractinellida, radial, or approximating to radial. The niicro- 

 scleres are, when present, some form of aster. The cortex 

 resembles that of Tetractinellida, and v. Lendenfeld has described 

 chones in Tethya lyncurium} 



The existence of the above points of resemblance between 

 Spintharophora and Tetractinellida suggests a relationship between 

 the two groups as its cause. In judging tliis possibility the follow- 

 ing reflections occur to us. A cortex exists in various independent 

 branches of Tetractinellida. It has in all probability had a different 

 phylogenetic history in each — why not then in these Monaxonida 

 also ? Within single genera of Tetractinellida some species are 

 corticate, others not, witness Tetilla. The value of a cortex for 

 purposes of classification may easily be overestimated. If we 

 are to uphold the relationship between these two groups, we 

 must base our argument on the conjunction of similar characters 

 in each. 



The genus Frotcleia " is interesting for its slender grapnel- 

 like spicules, which project beyond the radially disposed cortical 

 spicules, and simulate true anatriaenes of minute proportions. 

 That they are not anatriaenes is shown by the absence of an 

 axial thread in their cladi. It is not surprising that a form of 

 spicule of such obvious utility as the anatriaene should arise 

 more than once. 



Of exceptional interest, on account of their boring habit, are 

 the Clionidae. How the process of boring is effected is not 

 known ; the presence of an acid in the tissues was suspected, 

 but has been searched for in vain. The pieces of hard substance 

 removed by the activity of the sponge take their exit through 

 the osculum and have a fixed shape ^ (Fig. 108). 



As borers into oyster shells, Clionidae may be reckoned as pests 

 of practical importance, and in some coasts they even devastate the 

 rocks, penetrating to a depth of some feet, and causing them to 

 crumble away.* 



Sponges, however, as agents in altering the face of the earth 

 do not figure as destroyers merely. On the contrary, it has 



1 R. V. Lendenfeld, Acta Ac. German. Ixix. 1896, p. 22. 



2 Challenger Report, lix. 1887, p. 214. 



3 Topsent, Zoologie Descri2)tivc, i. ; also Cotte, C. R. Soc. Biol. Paris, 1902, 

 pp. 638-639. 



•* Topsent, Arch. Zool. Exp. (3) viii. 1900, p. 36. 



