PORIFERA. 



not in a true layer. In the cells of the mesenchyma spicules 

 are produced, forming the supporting skeleton (Fig. 77, C). 



4. Non-sexual reproduction is prevalent, but dimorphic 

 sexual cells are also formed in the mesenchyma. The sexually 

 produced larva is free-swimming; the adult is attached. 



5. Mostly marine; wholly aquatic. 



FIG. 76. Diagram of simple type of sponge, more mature than in Fig. 75. c,' cloaca; 

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

 mes., mesenchyma; o, osculum; r.c., radiating canals. Other letters as in Fig. 75. 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 figure 77. 



203. 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. 76), and 



