GRANTIA 183 
their relations to one another. Note the arrangement of the 
spicules which guard the incurrent pores, also of those tri- 
radiate spicules which project into the central cavity. 
Exercise 4. Make a drawing of several canals showing these 
features. 
Study a tangential section in which the canals appear in cross 
section and study the arrangement of the triradiate spicules 
around them. 
Exercise 5. Make a drawing illustrating it. 
Specialized reproductive organs are not present in Grantia. 
The sexual elements will be found in the form of large 
spherical bodies buried in the wall of the sponge. Fertiliza- 
tion takes place here, and development begins, and the young 
embryos escape into the sea water through the canals. For 
a while the embryo is a free-swimming animal, but it finally 
fastens itself to a rock and develops into the adult sponge. 
Besides this sexual reproduction, the sponge also reproduces 
asexually by budding. Each distinct cluster of individuals 
probably represents the gemmated progeny of a single indi- 
vidual. 
Special respiratory, excretory, digestive, circulatory, nervous, 
and locomotory organs are wanting in Grantia. Respiration and 
excretion are carried on through the entire surface of the body. 
The animal feeds on minute organisms and particles of organic 
matter suspended in the water which streams into the canal 
system through the incurrent pores. The radial canals are 
lined with peculiar entoderm cells called collar cells, each one of 
which possesses a flagellum. The action of the flagella pro- 
duces the current of water through the canals, from which the 
collar cells obtain and ingest food particles. Circulation is 
from cell to cell. 
