SPONGES. 113 



The animals that occupy the lower divisions of the scale 

 can exist only in a liquid element. Their forms present 

 many analogies with vegetables; and hence they have been 

 denominated Zooj)hytes, that is, animated plants: but as it 

 is now well ascertained that they possess the essential cha- 

 racters of animals, the term of Phytozoa. or plant-like ani- 

 mals, which has been given to them by some modern writers, 

 would appear to be a more appropriate designation. It is, 

 however, scarcely worth while at the present day, to change 

 a name so generally received as that of Zoophytes, and the 

 application of which is not likely to lead to any misunder- 

 standing. 



§ 2. Porifera, or Sponges. 



Among Zoophytes, the lowest station in the scale of or- 

 ganization is occupied by the tribes of Porifera^ the name 

 given by Dr. Grant to the animals which form the various 

 species of sponge, and which are met with in such multi- 

 tudes on every rocky coast of the ocean, from the shores of 

 Greenland to those of Australia. Sponges grow to a larger 

 size within the tropics, and are found to be more diminu- 

 tive, and of a firmer texture, as we approach the Polar cir- 

 cles. Dr. Grant observes* that they are met with equally 

 in places covered perpetually by the sea, as in those which 

 are left dry at every recess of the tide. They adhere to, 

 and spread over the surface of rocks and marine animals, to 

 which they are so firmly attached that they cannot be re- 

 moved without lacerating and injuring their bodies. "Al- 

 though they thrive best," he farther remarks, "in the shel- 

 tered cavities of rocks, they come to maturity in situations 

 exposed to 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 submarine caves, or hang in 

 living stalactites from the roof. " 



In their general appearance they resemble many kinds of 



* Edinburgh Philosophical Journal, vol. xiii. p. 94. 

 Vol. I. 15 



