356 IVILSOX. [Vol. IX. 



flagellated chambers. The afferent canals {af. c.) are spacious 

 cavities opening on the surface by wide mouths. The peri- 

 phery of the sponge forms a continuous rounded rim (the 

 " ringwall," ;-. w.), and the oscula, one or several, are situated 

 here. The surface of the sponge inside the ringwall is divided 

 up into low rounded elevations, caused by the upper ends of 

 the efferent canals, between which lie the wide apertures lead- 

 ing into the afferent canals. Schulze was fortunately able to 

 observe the main features in the development of this interest- 

 ing form. There is a solid swimming larva which settles 

 down, forming a fiat circular mass. A central cavity appears 

 in the mass, the lining cells becoming columnar, and the 

 sponge is thus transformed into a flat three-layered sac, PI. 

 XXV, Fig. 8, the three layers being respectively ectoderm, 

 mesoderm, and entoderm. The flagellated chambers appear in 

 a single layer round the central cavity, into which they open. 

 They are very probably formed as diverticula of this cavity. 

 Schulze did not follow the development further, but a compari- 

 son of the adult with the sac-like young form makes it pretty 

 certain that the young form undergoes a process of folding 

 which gives rise to the efferent and afferent canals of the 

 adult ; or in other words the efferent canals arise as vertical 

 evaginations of the sac-like stage. The afferent canals are 

 consequently to be regarded as lined with ectoderm. 



A vertical section of the second species, Plakina dilopha, is 

 shown in PL XXV, Fig. 9, and of the third species, Plakina 

 trilopha, in PI. XXV, Fig. 10. The oscula in these species are 

 not situated at the periphery as in Plakina monolopha, but at 

 some distance internal to it ; and in them the efferent canals 

 do not form projections on the surface as in the first species. 

 On comparing the canal systems in Figs. 7 and 9 it is seen 

 that Plakina dilopha has probably been derived from Plakina 

 monolopha by an increase in the thickness of the mesoderm 

 lying beneath the surface of the sponge. The wide afferent 

 canals of Plakina monolopha become transformed into the 

 narrow efferent canals of Plakina dilopha. In other respects 

 there has been no great change. (Schulze, pp. 438 and 



439-) 



