\ 
STRUCTURE OF SPONGES. 125 
agree with the Celentera, and differ from higher (trt- 
ploblastic and celomate) Metazoa. 
The body varies greatly in shape, even within the same 
species. It ts traversed by canals, through ° 
which currents of water bear food in- 
wards and waste outwards. Numerous 
minute pores on the surface open into 
afferent canals, leading into a cavity or 
cavities lined by flagellate cells, many or 
all of which have a goblet shape with a 
delicate collar through which the flag- 
ellum rises (‘‘choanocytes”). To the 
activity of the flagella the all-important 
water currents are due. The internal 
cavity may be a simple tube, or it may 
have radially outgrowing chambers, or tt 
may be represented by branched spaces, 
Jrom which afferent canals lead to the 
exterior. When there is a distinct central Fic. 56.—Simple 
cavity there ts usually but one large sponge (Ascetta 
exhalant aperture (osculum), but in other hig age 1 oe 
cases there are many exhalant apertures. ae 
A delicate outer layer covers the body, Nee aie aie 
and is perhaps continued into the affer- uaeaite pores in the 
ent canals. Beneath the covering layer ~*~ 
there is in all but the simplest forms a mass of cells (the 
mesoglea) which may be very varied in its composition. Thus 
there are scleroblasts making the skeleton of lime, flint, or 
Spongin ; amebotd cells or phago- 
cytes, important in digestion and 
excretion , reproductive cells, and 
other elements. 
This median mass of cells ts 
traversed by the afferent canals 
and by the diverticula of the 
central cavity or the branches of 
poecre the original central cavity, lined 
Fic. 57.—A sponge colony. Sy flagellate cells, It is difficult 
to call this cavity or system of 
cavities the gut or enteron, or to call the layer which lines it 
the endoderm, or the outer covering layer the ectoderm. In 
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