8 SPONGES’ 
PROVINCE IL. SPONGES. (PORIFERAE) 
Animals of a vegetable-like appearance, fixed when adult, free when 
very young. They are generally composed of two substances ; a hard, frequent- 
ly horny inner skeleton, and a softer outer covering, which is composed main- 
ly of a protoplasmic substance, which somewhat resembles flesh, and which is 
known as rudimentary flesh or sarcode. | Spicules, composed either of lime or 
of silica, are often embedded in this flesh. See remarks under groups of spong- 
eS: 
This flesh and skeleton is permeated by numerous tubes which are of 
three kinds, each of which has separate functions, but all of which form a water 
system. Oneset, smaller than the others, and which open externally, 
are incurrent tubes. Through these, the water is taken into the interior of the 
sponge, where it is taken up by the second set continuous with these and con- 
veyed to third largest set, the excurrent, which reach the outside, and from 
these the water is thrown out. 
The water.circulates freely through these ice which answ er for § a res- 
pratory system, and for the conveyance of food to the interior of the sponge. 
The water is induced to flow into the incurrent tubes by the motion of ci- 
lia, which are really false feet or pseudopoda, not unlike those seen in the 
Amoeba and other Inifusoria. “See Fig. 5, where I have given.a cut of an ideal. 
section of a cilia cell of’a sponge. Bis the incurrent tube; A the cilia, all 
with the cilia-hke pseudopoda. Compare this with those of the Amoeba, Fig. 
4, the projections being the psuedopoda. The water system, form of the cilia, 
ete. is more fully explained further on. . 
For difference’ between sponges and other closely allied animal organ- 
isms, see general conclusion at the end of the book. : 
