ANIMALCULES 



487 



The Bath-Sponge {Euspongid) possesses a horny skeleton, the 

 texture of which makes it useful for a variety of purposes. 



One small group of sponges is found in fresh water, and of 



Fig. 300. — Group of Calcareous Sponges i, Leucandra aspera, 2, Sycandra raphanus 



these the commonest British genus (Spongilla) is found as a 

 greenish crust upon various objects. 



ANIMALCULES (Protozoa) 



The innumerable host of simple animals which constitute 

 this phylum are nearly all minute, while many of them can only 

 be studied by the highest powers of the microscope, so that 

 every improvement in the construction of that instrument has 

 been followed by the acquisition of fresh knowledge concerning 

 this group. Very great interest attaches to the study of the 

 Protozoa, for here we have to deal with life under the simplest 

 conditions, and find the actual living substance, protoplasm, which 



