128 PHYLUM PORIFERA—SPONGES. 
nourished and multiply more rapidly than ‘those of the 
outer’ layer. 
(c) By infoldings of the outer layer and a subjacent 
sheath of mesoglcea—subdermal spaces may be formed; 
an outer cortex may be distinctly differentiated from the 
internal region in which the flagellate chambers occur; the 
pores may collect into sieve-like areas, which open into 
dome-like cavities; these and many other complications 
are common. 
(d) The covering layer usually consists of flat epithelium, 
but flask-shaped cells have also been observed (Bidder). 
It may be folded inwards, as we have noticed, and, accord- 
ing to some, it also lines the inhalant or afferent canals in 
whole or in part. In a few cases, e.g. Oscarella lobularis, 
it is ciliated, and its cells may also exhibit contractility, as 
around the osculum of Ascetfa clathrus, though the con- 
tractile elements usually belong to the mesoglcea. 
The inner layer consists typically of collared flagellate cells, 
but in the more complex sponges these are replaced, except 
in the flagellate chambers, by flat epithelial cells, with or 
without flagella. 
The mesoglea contains very varied elements, and illus- 
trates the beginnings of different kinds of tissue. Thus 
there are migrant amceboid cells (phagocytes); irregular 
connective tissue cells; spindle-shaped connective tissue 
cells, united into fibrous strands; contractile cells, e.g. 
those forming a sphincter around the oscula of some forms, 
such as Pachymatisma; skeleton-making cells; pigment- 
containing cells; supposed nerve cells, projecting on the 
surface, and believed to be connected internally with 
multipolar (ganglion?) cells; and lastly, the reproductive 
cells, 
(e) The skeleton consists of calcareous or siliceous 
spicules, or of spongin fibres, or of combinations of the 
two last. A calcareous spicule is formed of calcite, with a 
slight sheath and core of organic matter; a siliceous spicule 
is formed of colloid silica or opal; the spongin is chemically - 
somewhat like silk. Uniradiate, biradiate, triradiate, quadri- 
radiate, sexradiate, and multiradiate spicules occur, and 
they are effective in keeping the meshes open and in giving 
the body architectural stability. In every pole scaffolding 
