^4 



SPONGES. PHYLUM PORIFERA 



,ozc 



through the jelly, having their base in the covering layer 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. 

 Besides these cells of the dermal layer there are in the jelly 

 wandering amcrboid cells which appear, in some cases at least, 

 to belong neither to the gastral nor to the dermal layer, but to be 



descended independently from blasto- 

 meres 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. The current 

 which flows through the body is set up 

 by the working of the fiagella 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. 



-sp 



COMPLEX SPONGE BODIES 



Fig. 51. — The Olynthus 

 stage of a calcareous 

 sponge, from which a 

 portion of the wall has 

 been removed to expose 

 the paragaster. 



Osculum ; p., pores ; sp. 

 spicule in wall. 



osc 



No sponge remains at this simple stage 

 throughout its life. At the least the body 

 branches and thus complicates its shape, 

 and then often new oscula appear at the 

 ends of the branches. A higher grade is 

 reached when, as in the calcareous sponge 

 Sycon, the greater part of the vase is covered with blind, thimble- 

 shaped outgrowths, regularly arranged, and touching in places, but 

 leaving between them channels, known as inhalant canals, whose 

 openings on the surface of the sponge are often narrowed and are 

 known as ostia. The thimble-shaped chambers are known as 

 flagellated chambers, and are lined by choanocytes, but these 

 are now lacking from the paragaster, where they are replaced 

 by pinacocytes. Water enters by the ostia, passes along the 

 inhalant canals and through the pores, now known as prosopyles, 



