28 LIFE BY THE SEASHORE. 



ooze) which are at present accumulating on the floor of the 

 ocean. Protozoon shells may be found among shell-sand on 

 the rocks, but the Protozoon which is most likely to be 

 encountered without special search is the little Noctiluca, 

 the chief cause of the occasional li phosphorescence " of our 

 seas. It is just visible to the naked eye, and in the dark 

 appears like a tiny point of light. Into the characters of 

 the Protozoa we shall, however, not here enter in detail. 



The next great class of animals includes much more con- 

 spicuous forms the SPONGES, long thought to be plants. 

 The familiar bath sponge is, of course, merely the skeleton of 

 a once living animal, or rather of a collection of individuals, 

 a colony of sponges. For an example of a simple sponge 

 you should look under overhanging ledges of rock, and you 

 will find a little sac of dull colour and compressed form 

 hanging downwards. One end is fixed to the rock, the other 

 terminates in an opening which is not a mouth, for nothing 

 enters by it, but which serves as a means of exit for the 

 currents of water which enter the central cavity by numerous 

 pores in its walls. This central cavity is simple and un- 

 . divided ; there is no alimentary canal, and no organs, the 

 sponge is merely a thin-walled sac, lined with cells bearing 

 motile threads or cilia, which by their movement produce 

 continuous currents. Without power of locomotion, with but 

 little feeling, and no active means of defence, the sponges 

 would not be able to survive as they do were it not that they 

 are passively protected by their power of forming a skeleton. 

 This skeleton may be composed of sharp spicules of lime or 

 flint, or of silky fibres, as in the bath sponge, but in all 

 cases it seems to render the sponges ugly mouthfuls, and so 

 induces most animals to let them severely alone. In addition, 

 many sponges have a strong odour. Many are brightly 

 coloured. 



The little purse-sponge (Grantid), described above, has 

 usually a single large opening, .through which water leaves 

 the central cavity ; but many sponges, like the bath sponge, 

 or the very common crumb-of -bread sponge found on the 

 shore, have many of these large openings ; in the crumb-of- 

 bread sponge they stand up on the flat surface like little 

 craters. As each opening represents an individual, such 

 sponges are really colonies, formed by budding from an 



