No. 3.] DEVELOPMENT OF MARINE SPONGES. 335 
They become irregularly distributed through the body of the 
sponge. The short spicules (microscleres) likewise become 
distributed through the sponge body (Fig. 83), though the 
majority of them retain their peripheral location, many pro- 
jecting from the surface of the sponge, as shown in Figs. 84 
and 85. Sometimes a sponge is found with the peripheral 
microscleres arranged in as noticeably radial a fashion as in 
the swimming larva. Such an instance is shown in Pl. XXI, 
Fig. 86. Asarule the radial arrangement of the microscleres 
is not nearly so conspicuous as in this figure. At this period 
of its existence (Fig. 84), the sponge is a much simpler 
organism than during its swimming life, consisting as it does 
of a solid mass of parenchyma cells, in which there is no nice 
arrangement as in the free larva, and of an ectoderm which is 
nothing more than a nucleated membrane. 
Ectodermal Membrane and Peripheral Mesodermic Zone,— 
After attachment the edge of the sponge is for a time more or 
less circular and smooth, and the mesoderm extends quite to 
the periphery, Pl. XXI, Fig. 84, and Pl. XXII, Fig.92. The 
contour of the sponge then begins to change, and the periphery 
becomes more or less lobed and irregular, Pl. XXI, Fig. 86 
(compare also the ectodermal outline, ec., of the sponge given 
in Fig. 88). An accumulation of fluid then takes place in 
the extreme peripheral part of the sponge, by which means the 
ectodermal edge is pushed out some little distance from the 
edge of the mesoderm. In Fig. 88 this separation has taken 
place on opposite sides of the sponge, and between the edge of 
the mesoderm (#es.) and that of the ectoderm (ec.) is seen a 
clear space occupied by fluid alone. In Fig. 89 is represented 
a small part of the periphery of a sponge in which this process 
is going on—the parenchyma cells are separated from the 
ectoderm much farther in the middle than at the sides of the 
figure. In Pl. XXI, Fig. 87, a portion of the periphery of an- 
other sponge, fluid separates ectoderm from mesoderm in the 
regions a, 6, c. The ectoderm continues to grow peripherally, 
the distance between its edge and the edge of the mesoderm 
continually increasing. In this way the sponge body comes to 
be surrounded by a purely ectodermal membrane. In the 
