54 



INTRODUCTION TO ZOOLOGY 



CHAP. 



FIG. 28. Various forms of Sponge 

 Spicules (from Parker and Haswell). 



spicules which are so very characteristic of sponges and give 

 them their tough texture. These spicules are of very varied 



shapes in different sponges (Fig. 

 28); in those of the Ascon type 

 they are usually three-rayed and 

 calcareous. 



This very simple Ascon type 

 of structure is found in no adult 

 British sponge, but is a stage 

 passed through by some of them, 



e.g. Clathrina blanca, the White 

 T *.. d i i i 



I ^*? oe S POngC, which has at 



this stage a minute vase-like 

 body about ^-th of an inch high, similar in structure to that 

 shown in Fig. 27. 



Later the sponge branches in a complicated way, forming 

 a reticulate sponge body with several oscula. 

 S on T In Sycon, a British sponge to be found on the 



' south coast, there is still only one single central 

 chamber, but the wall is much 

 thicker, and from the central 

 cavity lateral branches extend 

 regularly and radially into the 

 wall ; the flagellate collar cells 

 are now restricted to these 

 radial extensions of the central 

 cavity which is itself lined 

 merely by flattened eudo- 

 dermal cells (Fig. 29). 



FIG. 29. Cross section through part 

 of the wal1 of a s y con - 



Incurrent canal ; the collar cells of the 

 radial chambers are shown by short 

 parallel lines (after Korschelt and 

 Heider). 



A further complication is 

 introduced by the outer skin 

 cells being pushed in between 

 two adjacent flagellate cham- 

 bers, so that the external 

 lateral pores open into a long 



narrow cavity which runs inwards between the flagellate 

 chambers, and communicates with them laterally (Fig. 29, a). 

 The water sucked in by the inhalent pores then passes down 

 an incurrent canal, then on into a flagellate chamber, and 

 finally into the central cavity and out of the osculum. 



In Sycons, as in Ascons, many spicules are present which 



