54 



INTRODUCTION TO ZOOLOGY 



Fig. 28. — Various forms of Sponge 

 Spicules (from Parlcer 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 

 Lattice Sponge, which has at 

 this stage a minute vase-like 



body about x^-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. 



In Syeon, 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 endo- 



dermal cells (Fig. 29). 



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 



Fig. 29. — Cross section through part 

 of the wall of a Sycon. 



a, iDCurrent canal ; the collar cells of the 

 radial chambers are shown by short 

 parallel lines (after Korschelt and 

 Heider). 



