148 THE MECHANICAL FUNCTIONS. 



the unbroken fury of the surge. They cover the 

 nakedness of cliffs and boulders ; they line with 

 a variegated and downy fleece the walls of sub- 

 marine caves, or hang in living stalactites from 

 the roof." 



In their general appearance they resemble 

 many kinds of plants, but in their internal or- 

 ganization they differ entirely from every vege- 

 table production ; being composed of a soft flesh, 

 intermixed with a tissue of fibres, some of which 

 are solid, others tubular ; and the whole being 

 interwoven together into a curious and compli- 

 cated net-work. The substance of which this 

 solid portion, or basis, is formed, is composed 

 partly of horn, and partly of siliceous or calcareous 

 matter. It has been termed the axis of the Zoo- 

 phyte ; and as it supports the softer substance 

 of the animal, it may be regarded as performing 

 the office of a skeleton, giving form and protec- 

 tion to the entire fabric. 



The material of which the fleshy portion is 

 composed is of so tender and gelatinous a nature 

 that the slightest pressure is sufficient to tear it 

 asunder, and allow the fluid parts to escape ; and 

 the whole soon melts away into a thin oily 

 liquid. When examined with the microscope 

 the soft flesh is seen to contain a great number 

 of minute grains, disseminated through a trans- 

 parent gelly. Every part of the surface of a 

 living sponge (as may be seen in Fig. 53) pre- 



