73 



Describe the Development of a Simple Ascon Sponge (e.g., 

 Leucosolenia). 



Segmentation results in an oval blastula of elongate 

 flagellate cells, all similar except two, which are large and 

 not flagellate. A number of the cells lose their flagella, 

 become amoeboid, and pass into the central cavity. This 

 larval form, with three kinds of cells, is the free- swimming 

 parenchymula. It becomes attached and flattened out, 

 and most of the inner amoeboid cells pass between the 

 flagellate ones to the outside and there form the outer 

 dermal layer. Of those that remain, some become poro- 

 cytes and pass to the outside where they hollow out to 

 form the inhalent openings of the wall, while the others 

 (which are archaeocytes developed from the two excep- 

 tional large non-flagellate cells) remain and become the 

 genital amcebocytes. 



Some of the cells of the dermal layer migrate inwards, 

 and are specialised as scleroblasts ; they group in threes 

 to form the triradiate spicules. 



The central cavity, lined by the flagellate cells; which 

 are now choanocytes, enlarges, and the adult form, with 

 an osculum at the free end, is attained. 



How does the Sycon Sponge differ in Development from the 

 simple Ascon Type ? 



The blastula has an upper hemisphere of small flagellate 

 cells and a lower hemisphere of large rounded archaeocytes. 

 The flagellate cells increase rapidly by division and soon 

 completely surround the archseocytes, now in the seg- 

 mentation cavity. This is the pseudogastrula stage. 



Then the flagellate cells of the lower hemisphere lose their 

 flagella and become large and rounded; and this larval 

 stage (with an upper hemisphere of flagellate cells and a 

 lower one of large round non-flagellate cells) is the am- 

 phi blastula which swims free fom the parent sponge and 

 then becomes fixed. 



The flagellate cells are overgrown by the large rounded 

 ones. The former become the choanocytes, and from the 



