104 PORIFERA. 
Fig. 38—Catcarrous Trt- acid. They are found to be 
RADIATE SPICULES OF calcareous in nature. They sup- 
SycanpRA (Grantia). hort the wall of the sponge and 
\ ‘form its skeleton. The further 
structure of Sycandra must be 
followed by prepared microscopic 
sections or by teasing to pieces 
and examination with the micro- 
scope. A transverse section as 
seen with low powers is shown 
in Fig. 39. The wall here shows 
Highly magnified. 
Fig. 39.—TRANSVERSE SECTION OF A SYCANDRA (A Sycon). 
yeuey syeyx” 
SE Inhalent Canal. 
The central cavity is the paragastric cavity. 
a number of vadial canals, some of them with thick 
walls and others with thin, The former open into 
the paragastric cavity by small contracted apertures and 
are called exhadent canals, whilst the latter open by the 
pores to the exterior and are termed ¢whalent canals. 
Further examination would show that the two sets of canals 
