METAZOA 37 
PorrFERA (the Sponges). 
CHARACTERISTICS.— Animals of very varied size and shape. 
Numerous minute pores allow the passage of water into the 
interior of the sponge, and the water is discharged through 
larger openings known as Oscula. The current of water 1s 
maintained by certain flagellate cells, which are usually 
aggregated in what are known as flagellate chambers. The 
mesoderm is well developed, and usually gives rise to a skeleton 
of calcareous, siliceous, or horny material ; it also gives origin 
to the reproductive cells. Sponges may be unisexual or 
hermaphrodite. They are aquatic, and, with the exception of 
the Spongillidae, they are marine. They are devoid of 
tentacles and of nematocysts. 
The simplest type of Sponge is that of Ascetta primordtalis, 
described by Haeckel. It is a hollow vase-like structure borne 
on a stalk with its free end open. This 
opening is the osculwm. The walls are per- 
forated by a series of small circular apertures, 
the “pores,” and its cavity is lined by a layer 
of flagellate collared cells, whose activity 
keeps a current of water entering the pores 
and finding an exit through the osculum. 
The flagellate cells are endodermal. The 
outside of the vase-like body is covered with 
ectoderm, and between these two layers is a 
mesodermic tissue which produces triradiate 
calcareous spicules. The flagellate endoderm 
cells are said to possess contractile vacuoles. 
In the more highly organised Sponges the 
endodermic lining of the central cavity has 
lost its flagellate character and become a 
flat epithelium. 
Grantia compressa 18 a sponge with a 
calcareous skeleton, which is frequently met 
with attached to rocks and stones round 
our coast. It is of a whitish colour, seldom 
Fig. 28.—Ascetta pri- 
mordialis, Haeckel. 
After Haeckel. 
more than an inch long, and rather variable in outline; a 
