118 THE MECHANICAL FUNCTIONS. 



Others have a kind of skeleton, composed of a tissue of 

 needle-shaped crystals of carbonate of li^e, 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 deli- 

 neated in Fig. 54: but their forms, although constant in each 

 species, admit of considerable diversity in the difierent 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 completely stationary as that of plants, yet 

 such is not the condition of the earlier, and more transitory 

 stages of their development. Nature, ever solicitous to pro- 

 vide for the multiplication of each race of beings, and for 

 . their dissemination over the habitable globe, has always pro- 

 ^ vided effectual means for the accomplishment of these im- 

 portant ends. The seeds of plants are either scattered in 

 the immediate neighbourhood of the parent, and take root 

 in the adjacent soil, or are carried to more distant situations 

 by the wind or other agents. In the animal kingdom, the 

 young offspring of those races which are endowed with a 

 wide range of activity, are reared on the spot where they 

 were produced, either by the fostering care of the parent, 

 or by means of the nourishment with which they are sur- 

 rounded in the egg, and there remain until the period when, 

 by the acquisition or extension of locomotive powers, they 

 are enabled, in their turn, to go in quest of food. But in the 

 tribes of animals at present under our consideration, this or- 

 der is reversed. It is the parent that is chained to the same 

 spot from an early period of its growth, and it is on the young 

 that the active powers of locomotion have been conferred, 

 apparently for the sole purpose of seeking for itself a proper 

 habitation at some distance from the place of its birth; and 

 when once it has made this selection, it there fixes itself un- 

 alterably for the remaining term of its existence."^ 



• Phenomena, which appear to bear some analogy with these, have been 



