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THE AMERICAN MUSEUM JOURNAL 



gastric cavity become folded in such a manner that the radial 

 tubes lose their radial position and open into the folds or their 

 branches (Fig. 5, cc). The openings by which the folds com- 

 municate with the paragastric cavity may then become narrowed 

 and thus large irregular spaces called exhalent lacuncB (exh.) 

 are formed, with the result that the radial tubes become mere 

 tubular chambers (cc.) communicating at the open end with the 

 paragastric cavity (atr.) only by the intervention of the inhalent 

 lacunas {inh.), which in turn commiinicate with the outside by 

 the pores. As both the inhalent and exhalent lacunas are lined 



FIG. 10. — A LARGE BAHAMAN SPONGE iHipposporgia cerebri/orniis D. & M.) 



with "pavement cells" {ect}) the choanocytes become restricted 

 to the tubular flagellate chambers, as the radial tubes are now 

 called. 



(c) The Rhagon Type (p. 221, Fig, 6). The two preceding 

 types of canal arrangement are peculiar to the sponges having a 

 calcareous or carbonate of lime skeleton. The great majority of 

 sponges, including those having " glass " skeletons, homy skeletons 

 or no skeletons at all, belong to the Rhagon type. In this case 

 the flagellate chambers {cc.) are very small and numerous and, 

 instead of being tubular, are spherical. The mesoderm varies 



