Fig. 41.—ASscETTA 
PRIMORDIALIS 
(IL&cKEL). 
A simple Ascon. Part of 
body-wall is removed to show 
Paragastric Cavity (X50). 
PORIFERA. 107 
PHYLUM PORIFERA. 
The PoriFERA or Sponges are a 
clearly-defined group. Their true re- 
lationship to other AZefazoa is not clear. 
They are evidently cell-aggregates with 
a large amount of physiological divi- 
sion of labour between the cells, and 
as they also have sexual reproduction 
they are undoubtedly A/efazoa. On 
the other hand, the sponges have no 
metazoan mouth nor anus — food is 
ingested by the cell-mouths of the 
choanocytes, so that ingestion, diges- 
tion, and egestion are purely intra- 
cellular. The cells are not aggregated 
into tissues and division of labour pre- 
vails more between individual cells 
than between epithelia of these cells. 
Hence sponges must be regarded as very 
simple cell-aggregates, belonging to the 
Metazoa. Most sponges are colonial, 
the colonies being produced by bud- 
ding. In many sponge-colonies the 
number of oscula alone indicates the 
theoretical number of individuals of 
which the colony consists. 
Fig. 42.—TRANSVERSE SECTION OF AN ASCON. (Diagrammatic.) 
Pinnacocytes. 
Choanocytes lining 
Paragastric Cavity. 
Pore. 
