342 



THE INFLUENCE OF LIVING SUEHOUNDING.?. 



stream of water, produced by the cells of the sponge, cir- 

 culates through the system of tubes thus formed, and this, it 

 would seem, supplies the animal with food, consisting of micro- 

 scopic organisms. By a course of growth and subdivision, 

 aftei' the manner of plant-growth, a compound sponge is fre- 

 quently formed ; one, that is to say, which has a number of 

 mouths, more or less, and in which the centi'al cavity — which 

 in calcareous sponges is often quite simple — is transformed into 

 a highly complex structure of internal canals and cavities. 



These soft and perfectly harmless organisms, sometimes, how- 



FlG. 91.— a. longitudinal section throuph a calcareous sponge, showing its simple central 

 cavity. 6, the sponge uninjiured. (From HaeckeL) 



ever, growing to an extraordinary size, offer a welcome shelter 

 in their inniimerable cavities to a host of other creatures, which 

 retire into them, as I might say, for rest and refreshment, and 

 cin easily find in their labyrinthine passages a j^lace of con- 

 cealment from the pursuit of their enemies; sometimes these 

 are true parasites, sometimes only commensals, which establish 

 themselves there. Such a specimen of sponge freshly dredged 

 up from the sea ofiers to the collector a rich mine of Annelids 

 aad Planarians, Nemertidse and Polypes ; Crabs of every kind, 

 various Mollusca, and even Fishes, may be found, and Plants, 



