No. 3-] DEVELOPMENT OF MARIAE 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. As a rule 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, PI. XXI, Fig. 84, and PI. XXII, Fig. 92. The 

 contour of the sponge then begins to change, and the periphery 

 becomes more or less lobed and irregular, PI. XXI, Fig. ?>6 

 (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 (jncs.) 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, b, 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 



