THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 33 



may become so interwoven with each other as to form a 

 very complex group. A sponge is composed of many cells 

 arranged in three layers that is, the body of a sponge is a 

 cylinder closed at one end whose wall is composed of three 

 layers of cells. The outer layer of 

 cells is called the ectoderm^ and the 

 cells composing it are flat and are 

 all closely attached to each other. 

 The inner layer is called the endo- 

 derm, and its cells are thicker than 

 those of the ectoderm ; they are 

 also closely attached to each other. 

 Sometimes they are provided with 

 flagella like the flagellate Protozoa. 

 The flagella are, however, not for the 

 purpose of locomotion, but for creat- 

 ing currents in the water, which 

 bathes the interior of the open cylin- 

 drical body. The middle layer, 

 called the mesoderm, is composed of 

 numerous separate cells lying in a 

 jelly-like matrix. From these meso- 

 derm cells fine needles or spicules 

 of lime or silica often project out 

 through the ectoderm. These mi- 

 nute sponge spicules are of a great 

 variety of shapes, and they form a 

 sort of skeleton for the support of 

 the soft body mass. All over the 

 outer surface of the body are scat- 

 tered fine openings or pores, which 

 lead through the walls of the body 

 into the inner cavity. This cavity is of course also con- 

 nected with the outside by the large opening at the free or 

 apical end of the body. 



There is hardly any differentiation of parts among the 

 4 



FIG. 17. One of the simplest 

 sponges, Calcolynthus pri- 

 migeniw (after HAECKEL). 

 A part of the outer wall is 

 cut away to show the in- 

 side. 



