1 68 



ZOOLOGY 



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 



»tts, 



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

 chambers, lined with flagellate cells; e.p., external pores; i.p,, internal pores; mw., mesenchyma; 

 o, osculum; i.e., incurrent 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 by the dermal 

 layer? With the gastral layer? What two main types of gastral cells are figured? 

 What is the actual nature of the mesenchyma 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 incurrent 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 



