356 WILSON. [Vou. 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,” 7. 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 flat 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 Pl. 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 g 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.) 
