126 SPONGES 



shape like a cake or bun, being usually slightly flattened and spread 

 out, with an irregular, but more or less circular outline. The upper 

 surface of the body is studded with minute pores (prosopyles), 

 leading directly into small rounded flagellated chambers, which in 

 their turn open by wide apopyles into a spacious gastral cavity, 

 lined everywhere by flattened epithelium. The water passes out 

 of the gastral cavity by the osculum, which is often raised up like 

 a chimney from the surface of the body. The lower surface of the 

 body is in contact with the surface of the object to which the 

 sponge is attached, and contains no chambers. Hence two regions 

 can be distinguished conveniently in the body wall ; a lower portion, 

 devoid of chambers or pores, the hypophare, and an upper portion, 

 containing all the chambers, the spongophare. 



From the foregoing it will be seen that the Rhagon is con- 

 siderably in advance of the Olynthus as regards organisation, since 

 it has a canal system of the second type, with the gastral layer 



FIG. 84. 



Vertical section of a Rhagon, diagrammatic, o, osculum ; p, gastral cavity. (After Keller, 

 X about 100). 



confined to the flagellated chambers, and the gastral cavity lined 

 everywhere by flat epithelium of the dermal layer. No stage with 

 fully formed pores and osculum, and with a canal system in a state 

 of functional activity, is known to occur of a simpler type than the 

 Rhagon in any Demosponge, but a transitory embryonic stage is 

 often found which may be interpreted as a suppressed and con- 

 tracted Olynthus stage (Fig. 63, ). No Demosponge is known, on 

 the other hand, which remains in the simple Rhagon condition ; 

 growth and folding of the wall lead in all cases to a series of pro- 

 gressive complications. 



The simplest adult type of canal system in Demospongiae is 

 represented by such a form as Plakina monolopha (Fig. 6 1,/), in 

 which the upper wall or spongophare of the primitive Rhagon has 

 become folded to form a number of lobes or diverticula. The 

 flagellated chambers become restricted to the walls of the diverticula 

 in question, and open into their cavities, which, though in origin 

 simply portions of a continuous gastral cavity, may be distinguished 

 conveniently as ezcurrent canals from the gastral cavity proper, just 

 as the spaces enclosed between the folds of the spongophare may 



