SPONGES 37 



or free end. Scattered over the surface of the sponge are 

 small rounded apertures, which are the openings of short 

 pores, that pass directly through the body walls into the 

 central or body cavity. These pores are called the in- 

 halent pores because the water flows through them 

 into the body cavity. From thence the water passes out 

 through the large opening; or osculum — exhalent pore — 

 at the free end of the sponge. Thus there is a continuous 

 circulation of water through the body of this simple animal. 

 The body wall consists of three distinct layers, an outer 

 layer, consisting of flat cells closely joined to each other 

 and covering the outside of the body; an inner layer, 

 of much thicker cells, lining the body cavity ; and between 

 these two, a layer of whitish, soft, gelatinous substance with 

 cells of various shapes imbedded in it. Each of the cells 

 composing the inner layer of tissue has a long, lashlike 

 appendage, called a fxtjellum, that projects from the end 

 of the cell into the large body cavity. These flagella are 

 constantly waving, or lashing, and in this way they main- 

 tain currents of water that bring bits of organic food to the 

 animal. All three layers of the body are soft, especially the 

 middle one, and if the sponge had nothing rigid to support 

 it, the body would coUapse. Imbedded in the middle, 

 gelatinous layer, are slender, glassy, needlelike bodies com- 

 posed of lime carbonate, and known as spicules. These 

 constitute the skeleton, as it were, and support the body, 



A More Complex Example of the Branch — the 



Simple Sponge, Grantia 



Distribution. — Grantia is a simple sponge found along 



the Atlantic and Pacific coasts. It occurs in small groups, 



attached to submerged rocks or other objects below low- 



