332 COMMON OBJECTS OF THE SHORE. 



as forming a link between the animal and vegetable 

 kingdoms. Since the day when that close observer, 

 John Ellis, a merchant of London, made accurate 

 observations on these productions, writers have 

 still varied in their opinions as to their nature, till 

 Dr. Grant satisfactorily proved that the marine 

 sponges, at least, are certainly animals. Dr. John- 

 ston enumerates fifty- six species of British sponge; 

 but these cannot be described merely by their ex- 

 ternal appearance, as the same species will often 

 assume a great variety of forms. In order to study 

 the sponges, we must look at them under a micro- 

 scope, and direct our attention to the nature of the 

 framework. Taking a small portion from the 

 common Mediterranean sponge of our toilet tables, 

 and placing it under a microscope, we see that it 

 is composed of a number of elastic horny fibres, 

 crossing each other in all directions, and forming 

 a complete network of meshes, through which the 

 water is absorbed, and passes 

 through every portion of the 

 sponge. Countless pores 

 cover the surface of this 

 network, and hence a cur- 

 rent of water goes towards 

 the centre whenever the 

 sponge is dipped in liquid. 

 But besides these minute 

 pores, there are large and 

 distant canals, having large 

 orifices on prominent portions of the body of the 

 sponge, and during the life of the animal the water 

 is, by means of some vital force, gradually collected 

 into these channels, and forcibly expelled through 



