ORDINARY FUNCTIONS. 129 
we see, as it were, huge hexactinellid spicules, spliced to- 
gether with ropé. It is convenient to distinguish the large 
macroscleres from the small microscleres. Each spicule 
begins to. be formed by one or more “scleroblasts,” and 
may be speculatively regarded as an organised intra- 
cellular excretion. ‘ During its growth,” Professor Sollas 
says, “the spicule slowly passes from the interior to the 
exterior of the sponge, and is finally (in at least some 
sponges—Geodia, Stelletta) cast out as an effete product.” 
The fibres of spongin are formed as the secretions of 
mesogloea cells, known as spongioblasts. 
Ordinary functions.—Excepting the fresh-water Spong- 
illide, all sponges are marine, occurring from between 
tide marks to great depths. After embryonic life is 
past, they live moored to rocks, shells, seaweeds, and 
the like. Their motor activity is almost completely 
restricted to the lashing movements of the flagella, the 
migrations of the phagocytes, and the contraction of 
muscular mesogloeal cells, especially around the exhalant 
apertures. In the closure of the inhalant pores, sponges 
show sensitiveness to injurious influences, but how far this 
is localised in specialised cells is uncertain. 
The most important fact in the life of a sponge is that 
which Robert Grant first observed—that currents of water 
pass gently in by the inhalant pores, and more forcibly 
out by the exhalant aperture or apertures. This may be 
demonstrated by adding powdered carmine to the water. 
‘The instreaming currents of water bear dissolved air and 
supplies of food, such as Infusorians, Diatoms, and particles 
of organic débris. The outflowing current carries away 
waste. When a sponge is fed with readily recognisable 
substances, such as carmine or milk, and afterwards 
sectioned, the grains or globules may be found—(a) in the 
collared flagellate cells; (4) in the adjacent phagocytes of 
the mesogloea ; (c) in the phagocytes surrounding the sub- 
dermal spaces, if these exist. It is uncertain whether the 
epithelium of the subdermal spaces or the flagellate lining 
of the deeper cavities is the more important area of absorp- 
tion, but it is certain that the phagocytes play an important 
part in engulfing and transporting particles, in digesting 
those which are useful, and in getting rid of the useless. 
9 
