THE SIMPLEST MANY-CELLED ANIMALS 323 



largest one. Also, some sponges, especially the green ones 

 which live in fresh water, form structures (called gemmules) 

 composed of cells well protected by a hard covering. These 

 gemmules may remain inactive until the following summer, 

 when they grow into new individuals. This kind of repro- 

 duction of sponges, like the budding, is asexual; but all 

 sponge-animals also have sexual reproduction. In each 

 animal both ova and sperm-cells are formed from certain 

 inner cells. The ova are usually fertilized by sperm-cells 

 produced and discharged into the water by other individuals 

 and carried into the canals by incoming currents of water. 

 The fertilized eggs develop into rounded embryos covered 

 with cilia and able to swim freely. After a time these settle 

 down on rocks, attach themselves by secreting a cement-like 

 substance, and grow into sponge-animals. Each animal 

 developed from an egg may later by branching, as described 

 above, form a large colony of sponges. 



The sponges of commerce (horny sponges) have the same 

 general plan of structure and life as described above for the 

 simplest sponges. The chief difference is that they branch 

 extensively and form immense colonies whose canals and cen- 

 tral cavities are united in a complicated network. An ordi- 

 nary large sponge is the skeleton of a colony of many sponge- 

 animals. Most of the large openings seen on such a sponge 

 are the united central cavities from a number of animals of 

 the colony, and the smaller openings are the united canals 

 for the ingoing of water to many individuals. Each individual 

 animal in such a complicated colony has a structure similar 

 to a simple sponge-animal. 



In preparing sponge skeletons for market, the flesh of the 

 masses (colonies) is allowed to depay by the action of bacteria, 

 which have no effect upon the skeletons. These are after- 

 wards cleaned by chemical solutions, which also sterilize them. 



Porifera. Because of the prominence of the pores for the 

 reception and expulsion of water, the sponge-animals are 



