PORIFERA OR SPONGES. 141 
work of horny fibres, strengthened by spicules of mineral 
matter, sometimes calcareous, but more commonly siliceous. 
The entire mass is traversed by a great number of canals, 
which may be said to commence in the small pores upon its 
surface, and which discharge themselves into the wide canals 
that terminate in the large orifices, or vents, that usually pro- 
lect more or less from the surface 
of the Sponge. Through this sys- 
em of canals, there is continually 
faking place, during the living state 
of the animal, a circulation of water, 
which is drawn in from without 
through the minute pores, then 
passes into the large canals, and is 
gjected in a constant stream from 
the vents. The immediate cause of this movement seems 
0 lie in the vibration of cilia so extremely minute that their 
sxistence can only be detected by the most careful micro- 
scopic examination. Its purpose is evidently to convey to 
the animal the nutriment which it requires, and to carry off 
the matter which it has to reject. No distinct indications of 
sensation, or of power of locomotion, have been seen in the 
Sponge: but changes in the form of its projecting vents 
may be seen to take place from time to time, if it be watched 
sufficiently long. 
_ 137. The reproduction of the Sponge is commonly effected 
ay the budding forth of little particles of sarcode, from the 
ayer which lines the larger canals ; these become furnished 
vith cilia, and, when detached and carried out by the current 
hat issues from the vents, swim freely about for some time ; 
io as, before fixing themselves and beginning to develope 
nto Sponges, to spread the race through distant localities. 
But it appears that Sponges are also reproduced by a true 
exual process ; “sperm-cells” and “ germ-cells” being pro- 
luced (as in the Hydra, § 123) in different parts of the 
Mganism, and a true embryo taking its origin in the action 
Mf the contents of the former upon those of the latter. 
Fig. 81.—Spronce. 
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