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. 



