lOO AN INTRODUCTION TO THE STUDY OF FOSSILS 



to the walls by a network of interlacing calcareous spicules. 

 These are microscopic, spike-like calcareous bodies, with one to 

 four rays. Each spicule is secreted by a single mesogloeal cell, 

 the remains of the cell being at times still distinguishable as a 

 thin investment upon the surface of the full-grown spicule. 



Food and Digestion. — The entire surface of the sponge is 

 perforated by numerous, minute holes (ostia), through which 

 a current of water enters the incurrent canals (lined with 

 flattened ectodermal cells) ; from here it enters the elongate 

 flagellate canals and passes out into the large central, or 

 paragastric cavity by the short excurrent canals ; these elongate 

 flagellate or gastral canals are lined with endodermal cells, each 

 of which is furnished with a protoplasmic collar from within 

 which projects a thread-like lash, the flagellum ; these cells are 

 very similar to certain Protozoa, — the Choanoflagellata, of 

 the class Mastigophora. These flagella, having a stronger and 

 swifter movement in one direction, moving in some forms ten 

 times a second, cause the currents of water to pass from the 

 outside through the incurrent, flagellate, and excurrent canals 

 into the large central or paragastric cavity and thence out 

 through the large opening, — the osculum, at the summit of 

 the sponge. The excurrent canal is in Grantia very short, a 

 mere opening ; in all the higher sponges, as in Spongilla, it 

 is very long. This current of water carries with it many mi- 

 croscopic animals and plants, and food particles, which the 

 collar-cells capture and ingest by means of the flagella and col- 

 lars ; each cell acts thus as a single protozoon. Wandering 

 amoeba-form cells take part in digestion ; these can move 

 from one part of the sponge to another. Thus digestion usually 

 takes place within individual cells as in the Protozoa. 



Circulation. — The transference of the digested particles of 

 food from the digestive cells to the rest of the body is largely a 

 process of simple osmosis, — the cells having recently digested 

 food being denser than the others ; it is aided also by the very 

 free wandering of the amceba-form cells and to a less extent by 



