154 THE MECHANICAL FUNCTIONS. 



would lead us to ascribe them to the action of 

 fibrils, or cilia, as they are termed, projecting 

 from the sides of the canals through which the 

 streams pass ; but these cilia have hitherto 

 eluded observation, even with the highest powers 

 of the microscope. 



The organization of sponges is as regular and 

 determinate as that of any other animal struc- 

 ture, and presents as systematic an arrangement 

 of parts. In some species, such as the common 

 sponge, the basis is horny and elastic, and com- 

 posed of cylindric tubes, which open into each 

 other, and thus form continuous canals through- 

 out the whole mass. 



Others have a kind of skeleton, composed of 

 a tissue of needle-shaped crystals of carbonate 

 of lime, or of silex. These hard and sharp- 

 pointed fibres, or spicula, are disposed around 

 the internal canals of the sponge, in the order 

 best calculated to defend them from compression, 

 and from the entrance of foreign bodies. Some 

 of these spicula are delineated in Fig. 54 : but 

 their forms, although constant in each species, 

 admit of considerable diversity in the different 

 kinds of sponge. 



Although sponges, in common with the greater 

 number of zoophytes, are permanently attached 

 to rocks, and other solid bodies in the ocean, 

 and are consequently destined to an existence 



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