PORIFERA 



I6 7 



5. Mostly marine; wholly aquatic. 



208. General Form. The simpler sponges are cylindrical 

 or vase-shaped sacs with an opening (the osculum) at the un- 

 attached end. From the central cavity (cloaca) of the sac 

 numerous radial passages pierce the walls (Fig. 78), and 

 terminate directly or indirectly in pores at the surface (whence 



_E.Jb. 





FIG. 78. Diagram of simple type of sponge, more mature than in Fig. 77. c, cloaca; ch, 

 chambers, lined with flagellate entoderm; e.p., external pores; i.p., internal pores; mcs., mesenchyma; 

 o, osculum; r.c., radiating canals. Other letters as in Fig. 77. In .the adult sponge the canals and 

 flagellate chambers become much more complex than figured here. 



Questions on the figure. What portions of the animal are lined with ectoderm? 

 With entoderm? What two main types of entoderm are figured? What is the 

 actual nature of the mesoderm in sponges? Is there a coelom (a cavity bounded by 

 mesoderm) ? What mechanical advantage do you see in the fact that the water 

 currents enter by way of the radial canals and find their exit through the osculum, 

 rather than the opposite direction? Compare with Pig. 79. 



the name Porifera). In the more complicated sponges there 

 is such power of budding and lateral growth that there is 

 formed a dense tuft of sponge made up of many individuals 

 in organic connection with each other. In such sponges the 

 simplicity of the internal structure is lost, and the cloaca may 

 branch, opening to the exterior by a number of oscula. The 

 radial passages which penetrate the wall become much branched 



