PORIFERA. 



77 



by flat cells, together with a reduction in length of the flagellated 

 chambers (Leucilla uter, Leucandra aspera, etc.). Finally the 

 chambers are small and spherical and irregularly scattered through 

 the sponge- wall; the inhalent and exhalent canals being fljuch 

 developed. This is the Leucon stage of Haeckel and is found in 

 Leucandra, Leucilla, etc. In the non-calcareous sponges the canal- 

 system is generally on a somewhat different plan. The simplest 

 form is the so-called Rhagon type (Fig. 66) found in the embryos of 

 certain forms (Plakina, Reniera). The Ehagon has the form of a 

 flattened pyramid, attached by its broad base and opening by the 

 osculum at its apex. The sac is lined by flat cells, but possesses 

 on its upper wall a number of small flagellated chambers into which 

 the prosopyles open. The lower basal wall of the Rhagon, which is 



FIG. 67. Diagram showing the relations of the ectosome. H hypophare; E ectosome ; 

 I fold of the ectosome roofing over the incurrent sinus ; p-& sieve-plate of dermal ostia 

 or inhalent pores ; g excurrent sinuses ; v flagellated chambers ; S incurrent sinuses (from 

 Perrier). 



without flagellated chambers, is called the hypophare, and the upper 

 wall, with the chambers, the spongophare. The openings of the 

 chambers into the gastric cavity of the Ehagon are called the 

 apopyles. The Rhagon condition is not found in any adult sponge, 

 but the nearest approach to it is presented by Plakina monolopha 

 and Oscarella lobularis, in which the spongophare is folded, so as 

 to give rise to incurrent sinuses or canals similar to the intercanals 

 of the Calcarea (Fig. 66, 2). 



As a result of this same folding the chambers open, not into the 

 central or gastral cavity, but into diverticula of it. These diverticula 

 are the excurrent sinuses. We thus get a modification of the Rhagon 

 canal-system called the Eurypylous type. As a general rule in the 

 Eurypylous type, there is concrescence between the folds of the 

 spongophare, and the openings of the incurrent sinuses are roofed 



