38 THE president's address. 



various different purposes, such as the protection of the dermal 

 surface by discotriaenes and the anchoring of sponges which 

 live on a muddy bottom by means of " grapnel spicules." 



As regards such adaptive modifications it really seems as if 

 the sponge were able to do anything that may be required with 

 the inherited material at its disposal, to convert a calthrops into 

 a pitchfork, a grapnel or an armour plate as occasion demands. 



There are, however, as I have pointed out to you before, other 

 series of skeletal modifications which seem to have no adaptive 

 significance whatever. These are perhaps best seen in the 

 extraordinary forms assumed by so many of the microscleres in 

 the Tetraxonida, all of which may be derived, through more 

 or less complete series of known intermediates, from the primi- 

 tive tetract. It is quite impossible, for example, to assign any 

 useful purpose to those extraordinary and singularly beautiful 

 spicules which we call chelae, so characteristic of the family 

 Desmacidonidae. Yet these chelae exhibit such definite and 

 constant forms, and at the same time occur under so many 

 different modifications, that they afford some of the most 

 useful criteria of specific distinction in this group. The reason 

 for the existence of their divers modifications is at present 

 a complete mystery to us ; we can only say that they must 

 be, to use a mathematical expression, functions of heritable 

 modifications in the constitution of the protoplasm of the 

 germ-cells, the nature of which is as yet beyond reach of 

 investigation, and that they are perhaps correlated with other 

 characters which are of real value to the sponge.* It seems 

 to me probable that they originate in most, if not in all, cases 

 as mutations, for they seem to be just the sort of characters 

 that are known to originate in this way in other groups of 

 the animal kingdom ; but there is, I think, better evidence 

 of the occurrence of mutations in sponges, though unfortunately 

 it is as yet impossible to put the matter to the test of breeding 

 experiments. 



In attempting to draw up a scheme of classification, whether 

 it be of the Calcarea or of the Tetraxonida, we find certain very 



* Cf. my previous address on "By-Product j of Organic Evolution," 

 Journ. Q.M.C., vol. xii. p. 05. 



