SPONGES 



marked are inhabitants of shallow water ; or, if not, they are 

 forms whose nearest allies are to be found along the shore, and 

 whose ancestors have probably migrated into deeper water in com- 

 paratively recent times. In other words, the " impersonal " con- 

 dition, as it may be termed, seems to have been correlated at its 

 first origin with life in a habitat where the sponge has to contend 

 with, and to adapt itself to, the action of stresses and strains which 

 are always very variable and often very severe, and where the 



osc. 



st. 



Fio. 33. 



Spongilla lacustris, auct. (after 

 Weltner). J. 



Fio. 34. 



Chalina oculuta, Pall, half-natural sizo. 

 osc, oscula ; st, stalk. 



form of the sponge becomes of the greatest importance in the 

 struggle for existence. Hence the sponge colony as a whole takes 

 on some characteristic mode of growth which may vary greatly 

 from species to species, or even in different specimens of the same 

 species. In this way a great number of different shapes and forms 

 arise which are often extremely irregular and amorphous, but which 

 can usually be classed under one of a series of typical forms. 

 As the starting-point we may conveniently take a small com- 

 pact sponge with numerous oscula that is to say, a colony in which 



