PORIFERA. 165 



entoderm. Tubes lined with entoderm pass out from this central 

 cavity through, or into, the body wall. These tubes may enlarge 

 into chambers (Fig. 42, ck) also lined with entoderm. Into the 

 outer part of these chambers tubes lined with ectoderm enters. It is 

 through these passages that the water, bearing food and oxygen, 



,0 



FIG. 42. Diagram of a simple type of sponge, c, cloaca; ch, chambers, lined with 

 flagellate entoderm; e.p., external pores; i.p., internal pores; wes, mesenchyma; o, 

 osculum; r.c., radiating canals. 



Questions on the Figure. What portions of the animal are lined 

 with entoderm? With ectoderm? What two main types of ento- 

 derm are figured? What is the actual character of the mesoderm 

 in sponges? Is there a ccelom (. e., a cavity bounded by mesoderm) ? 

 Why is it better for the water to pass in through the pores and out 

 through the osculum, rather than the other way? Compare with 

 Fig. 43. Fig. 42 is merely a later stage of Fig. 43, 5. What changes 

 have taken place? 



enters the animal. The united currents, bearing the wastes of the 

 body, pass out through the larger opening (osculum; Fig. 42, o). 



The body wall is compased (i) of the outer layer of cells (ectoderm), 

 the inner layer of cells (entoderm), and (3) between these a gelatinous 

 secretion in which there are several kinds of cells (mesoderm; Fig. 42, 

 mes). There are cells in the mesoderm which secrete the skeleton; 



