140 THE MECHANICAL FUNCTIONS. 



through the numerous channels of communication 

 which pervade the substance of the body, it is col- 

 lected into wider passages, terminating in the fecal 

 orifices above described, and is finally discharged. 

 The mechanism by which these currents are pro- 

 duced is involved in much obscurity. The analogy 

 of other zoophytes would lead us to ascribe them 

 to the action of cilia, 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 structure, 

 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 throughout 

 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 com- 

 pression, and prevent the entrance of foreign bodies. 

 Some of these spicula are delineated in Fig. 54: 

 but their forms, though 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 as com- 

 pletely stationary as that of plants, yet such is not 



