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 mouths, 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, 29). 



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, and nearly the whole 

 mass, as seen in the common sponge, consists of spicules or 

 horny fibres, and protoplasm, through which the excurrent 

 and incurreiit 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 gantrula are used in this boos simply tor 



