PHYSIOLOGY DISTRIBUTION 



239 



lip the strands of spicules as they formed 

 them, leaving the lower parts behind and 

 adding to the upper ends. 

 , Of the physiology of the pigments of 

 sponges not much is yet known : a useful 

 summary of facts will be found in Von 

 Fiirth's text-book.'' 



Spongin. — Von Fiirth ^ points out that 

 this term is really a collective one, seeing 

 that the identity of the organic skeletal 

 substance of all sponge species is hardly to 

 be assumed. Spongin is remarkable for 

 containing iodine. The amount of iodine 

 present in different sponges varies widely, 

 reaching in certain tropical species of the 

 Aplysinidae and Spongidae the high figure 

 8 to 14 per cent. Seaweeds which are 

 specially rich in iodine contain only 1'5 

 to 1'6 per cent. 



In view of the fact that iodine is a 

 specific for croup, it is of interest to observe 

 that the old herb doctors for many centuries 

 recognised the bath sponge as a cure for 

 that disease. 



Distribution in Space. — All the larger 

 groups of Sponges are cosmopolitan. Each 

 group has, however, its characteristic bathy- 

 metrical range : the facts are best displayed 

 by means of curves, as in Pig. 122, which 

 is based wholly on the results obtained by 

 the " Challenger " Expedition. The in- 

 formation as to littoral species is con- 

 sequently inadequate, and we have not the 

 data requisite for their discussion. 



Sponges generally (a) and Monaxonida 

 in particular (b) are more generally dis- 

 tributed in water of depths of 51 to 200 

 fathoms than in depths of less than 50 

 fathoms ; but localities in shallow water are 



1 Fergl. Physiologie d. niederen Thiere, 



I. II. 



FiQ. 122. — The ordinals mea- 

 sure (i.) the number of 

 species, a-/, and (ii.) the 

 number of stations, a'-f, 

 at which successful hauls 

 were made. The abscissae 

 measure the depth : thus 

 at I. the depth is from 

 to 50 fathoms ; at IE. 

 from 51 to 200 ; at III. 

 from 201 to 1000 ; at IV. 

 froni 1001 upwards, a, a, 

 are the curves for Sponges 

 generally ; 6, V, for Mon- 

 axonida; c.c', for Hexacti- 

 nellida ; d, d', for Tetrac- 

 tinellida ; e, c', for Cal- 

 carea; /,/', for Ceratosa. 



1903, p. 441. 



