ii8 



THE INVERTEBRATA 



of scattered cells, with a jelly in which they are imbedded. The most 

 numerous of these cells are engaged in secreting spicules of calcium 

 carbonate by which the wall is supported. They wander from the 

 covering layer into the jelly, and then each divides into two, and the 

 resulting pair secrete in their protoplasm, which is continuous, a 

 needle-like spicule which presently outgrows them. Most often the 

 original spicule cells come together in threes before this process, so 

 that the three spicules which they secrete become the rays of a three- 

 rayed compound spicule. This lies in the wall with two rays towards 

 the osculum and one away from it. Sometimes a fourth cell joins the 

 others later, and forms a fourth ray which projects inwards towards 

 the paragaster. Often there are simple spicules which project from 

 the surface of the sponge. Other cells, known asporocytes, of a conical 

 shape, extend through the jelly, having their base in the covering layer 



am 



Fig. 98. Part of a longitudinal section of the wall of an Olynthus, including 

 a portion of the rim of the osculum. From Borradaile. a.m. amoeboid cell ; 

 ch. choanocyte; e.' flat covering cells (pinacocytes) of dermal layer; e.'' similar 

 cells lining the rim of the osculum ;;. jelly; por. pore; pc. young porocyte; 

 pc' fully developed porocyte; sp. spicule; sp.c. spicule cell. 



while their apex reaches the paragaster between the choanocytes. Each 

 is pierced from base to apex by a tube, which is one of the pores. Be- 

 sides these cells of the dermal layer, there are in the jelly wandering 

 amoeboid cells which appear, in some cases at least, to belong neither 

 to the gastral nor to the dermal layer, but to be descended inde- 

 pendently from blastomeres of the embryo. Some of them become 

 ova; others, it is believed, give rise to male gametes; the rest are 

 occupied in transporting nutriment and excreta about the sponge. 

 There are no nerve or sense cells in this or any other sponge. 



The current which flows through the body is set up by the working 

 of the flagella of the choanocytes. It carries with it various minute 

 organisms which serve the sponge for food, being swallowed, in some 

 way which is still in dispute, by the collar cells. These digest the food, 

 rejecting the indigestible parts into the space within the collar; and 

 passing on the digested food to amoebocytes, which visit them to 

 obtain it. 



