124 



ZOOLOGY 



SECT. 



perforated by apertures the inhalant apertures in the wall of 

 the sponge. Among the flagellate cells and porocytes there are 

 also amoeboid cells derived from the two original granular cells ; 



some o f these 

 give rise to the re- 

 productive cells. 

 The scleroblasts 

 are formed of 

 certain ectoderm 

 cells which 

 migrate inwards, 

 and at an early 

 stage arrange 

 themselves i n 

 threes to give 

 rise to the tri- 

 radiate spicules. 

 The development 

 of the sponge be- 

 comes completed 

 by the enlarge- 

 ment of the in- 

 ternal cavity 

 (paragastric 

 cavity) which is 

 now lined b y 

 flagellate cells, 

 and by the de- 

 velopment of the 

 osculum. 



In Sycon the 

 early stages 

 (Fig. 95, a-e) 

 differ somewhat 

 from those in 

 Clathrina blanca, 

 and the embryo 

 leaves the parent 

 sponge in the 

 peculiar stage to 

 which the name 

 of amphiblasiula 

 is applied. When 

 the blastula i s 



formed the greater part of its wall consists of clear cells, with a 

 number of granular cells the archaeocytes at the posterior pole. 



