STRUCTURE OF SPONGES. 43 
jects its own waste particle of food, the protoplasm having 
been previously absorbed, and the waste from all the epi- 
thelial cells is collectively expelled from the single excurrent 
orifice (osculum), there being many pores or ee and 
but a single outlet for the rejectamenta. 
Such is the structure of one of the simplest sponges ; the 
larger common sponges differ mainly in having a less defi- 
nite form, with numerous sacs or digestive cavities or cham- 
bers, and numerous excurrent orifices or oscula. It will be 
‘seen, then, that we have in the sponge a three-layered sac, 
its cavity rudely foreshadowing the gastrovascular cavity of 
the Hydra, but with no genuine mouth, the pores or so- 
called mouths simply allowing the sea-water laden with 
sponge-food to flow in, inflowing currents being formed by 
the ciliary action of the digestive cells, and the excurrent 
orifice permitting its exit (Figs. 29, 29a). 
In the other sponges such as are figured in this chapter, 
the structure is a little more complicated than in the 
Ascetta. There is no general body-cavity, with a contin- 
uous lining of epithelial cells, but the entire sponge-mass is 
‘permeated by large canals ending in oscula, and there are 
innumerable pores (so-called mouths) leading by branching 
‘canals to little pockets or cavities, which are lined with the 
flagellate, collared cells developed specially from the inner 
cell-layer (endoderm) ; so that the animal is myriad-stom- 
ached, so to speak. Moreover, the middle layer of cells is in 
many sponges greatly thickened. und nearly the whole 
Mass, as seen in the common sponge, consists of spicules or 
horny fibres, and protoplasm, through which the excurrent 
and incurrent channels meander. Thread. cells or lasso- 
cells like those hereafter to be described in Hydra have 
been detected in the sponge named Reniera. 
Let us now follow out the life-history of a sponge. The 
sponges are further distinguished from the Protozoa in pro- 
ducing eggs and spermatic particles, the eggs being fertilized 
before leaving the sponge. The egg after fertilization di- 
vides in two, four, eight, sixteen, and more spheres, attain- 
ing the mulberry or morula * state (Fig. 30). The result is 
* The terms morula and gastrula are used in this book simply for 
