2O ANIMAL BIOLOGY 



spongin, a durable substance of the same chemical nature as silk 



(Figs. 30 and 31). 



The limy sponges have skeletons made of numerous spicules of 



lime. The three-rayed spicule is the commonest form. 



The commercial sponge, seen as it grows in the ocean, appears- 



as a roundish mass with a smooth, dark exterior, and having about; 



the consistency of beef liver. Several large openings (oscula),, 



from which the water flows, are visible on the upper surface. 



Smaller holes (inhalent pores many of them so small as to be 



indistinguishable) are on the sides. If the sponge is disturbed,. 



the smaller holes, and 

 perhaps the larger 

 ones, will close. 



The outer layer of 

 cells serves as a sort 

 of skin. Since so> 

 much of the sponge 

 is in contact with, 

 water, most of the 

 FIG. 30. A homy FIG. 31. -Section ce u s <j o their own 



sponge. of horny sponge. . 



breathing, or absorp- 

 tion of oxygen and giving off of carbon dioxid. Nutriment is. 

 passed on from the surface cells to nourish the rest of the body. 



Reproduction. Egg-cells and sperm-cells are produced by 

 certain cells along the canals. The egg-cell, after it is fertilized 

 by the sperm-cell, begins to divide and form new cells, some of 

 which possess cilia. The embryo sponge passes out at an oscu- 

 lum. By the vibration of the cilia, it swims about for a while. 

 It afterwards settles down with the one end attached to the ocean 

 floor and remains fixed for the rest of its life. The other end de- 

 velops oscula. Some of the cilia continue to vibrate and create 

 currents which bring food and oxygen. 



The cilia in many species are found only in cavities called 

 ciliated chambers. (Figs. 22, 32.) There are no distinct organs 

 in the sponge and there is very little specialization of cells. The 

 ciliated cells and the reproductive cells are the only specialized 

 cells. The sponges were for a long time considered as colonies 

 of separate one-celled animals classed as protozoans. They are, 



