116 THE SEA SHORE 



In order to ascertain the general features of a sponge we cannot 



do better than select one of the simplest forms from our own shores. 



If we place the live animal in a glass vessel of sea water, and 



examine it with a suitable magnifying power, we observe a 



number of minute pores scattered over its whole surface ; and a 



much larger opening at the free end. The animal is motionless, 



and exhibits no signs of life except that it may contract slightly 



when touched. The water surrounding the sponge also appears to 



be perfectly still, but if we introduce some fine insoluble powder, 



, such as precipitated chalk, or a drop of 



<$k ' 8^ a som ble dye, the motion of the sus- 



/$? ^H^ pended or soluble material will show 



fff -^ that the water is passing into the 



2> sponge through all the small pores, and 



that it is ejected through the larger 



~* Pi jj opening. 



set On touching the sponge we observe 



Hi that it is of a soft, gelatinous consistence 



"""* Hi H throughout, or if, as is often the case, the 



jSji body is supported by a skeleton of greater 



yp ^f or less firmness, a gentle application of 



Jf9 the finger will still show that this frame- 



J~ work is surrounded by material of a jelly - 



\M ll? like nature. This gelatinous substance 



tj|? is the animal itself, and a microscopic 



Jjj examination will show that its body- wall 



is made up of two distinct layers, the 

 inner consisting of cells, many of which 

 == possess a cilium or whip-like filament 



FIG. 65. SECTION OF A that protrudes from a kind of collar, its 

 SIMPLE SPONGE free extremity extending into the body- 



cavity. 



These minute cilia are the means by which the water currents 

 just described are set up. By a constant lashing movement they 

 urge the fluid contained in the body-cavity towards the larger hole, 

 thus causing the water to flow in through the numerous small 

 pores. This circulation of sea water through the body-cavity of 

 the sponge is the means by which the animal is supplied with air 

 and food. Air is, of course, absorbed from the water by the soft 

 material of the external layer of the body, but the constant 

 flow of fresh water through the body-cavity enables this process of 



