CHAPTER XV 



BACKBONELESS ANIMALS 



I. Sponges — 2. Stinging-Animals or Cxlenterata — 3. " Worms" — 

 4. Echinoderms — 5. Arthropods — 6. Molluscs 



I. SpongeS.^ — Sponges are many -celled animals without organs, 

 with little division of labour among their cells. A true "body" is 

 only beginning among sponges. 



Adult sponges are sedentary, and plant -like in their growth. 

 With the exception of the freshwater sponge (Spongilld) they live 

 in the sea fixed to the rocks, to seaweeds and to animals, or to the 

 muddy bottom at slight or at great depths. They feed on micro- 

 scopic organisms and particles, bonie in with currents of water 

 which continually flow through the sponge. The sponge is a 

 Venice-like city of cells, penetrated by canals, in which incoming 

 and outflowing currents are kept up by the lashing activity of 

 internal ciliated cells. These ciliated cells, on which the whole life 

 of the sponge depends, line the canals, but are especially developed 

 in little clusters or ciliated chambers. The currents are drawn in 

 through very small pores all over the surface ; they usually flow out 

 through much larger crater-like openings. 



Sponges feed easily and well, and many of them grow out in 

 buds and branches. A form which was at first a' simple cup may 

 grow into a broad disc or into a tree-like system. And as trees are 

 blown out of shape by the wind, so sponges are influenced by the 

 currents which play around them, as well as by the nature of the 

 objects on which they are fixed. Like many other passive 

 organisms, sponges almost always have a well-developed skeleton, 

 made of flinty needles and threads, of spicules of lime, or of fibres 

 of horn -like stuff. While sponges do not rise high in organic 

 rank, they have many internal complications and much beauty. 



Sponges may be classified according to their skeleton, as 



