PORIFERA. 



107 



Fig. 41.-ASCETTA PHYLUM PORIFERA. 



Primordialis ^, ^ ^ 



(H.^xkel). ^he PoRiFERA or Sponges are a 



clearly-defined group. Their true re- 

 lationship to qI\\^x Metazoa is not clear. 

 They are evidently cell-aggregates with 

 a large amount of physiological divi- 

 sion of labour between the cells, and 

 as they also have sexual reproduction 

 they are undoubtedly Metazoa. On 

 the other hand, the sponges have no 

 metazoan mouth nor anus — food is 

 ingested by the cell-mouths of the 

 choanocytes, so that ingestion, diges- 

 tion, and egestion are purely intra- 

 cellular. The cells are not aggregated 

 into tissues and division of labour pre- 

 vails more between individual cells 

 than between epithelia of these cells. 

 Hence sponges must be regarded as very 

 simple cell-aggregates^ belonging to the 

 Metazoa, Most sponges are colonial, 

 the colonies being produced by bud- 

 ding. In many sponge-colonies the 

 number of oscula alone indicates the 

 A simple Ascon. Part of thcoretical numbcr of individuals of 



body- wall is removed to show 1 • 1 ^1 i 



Paragastric Cavity (X 50). whlch the Colouy COUSlStS. 



Fig. 42. — Transverse Section of an Ascon. (Diagrammatic.) 



Mesogloea 



Pinnacocytes. 



■* Choanocytes lining 



. ^ \ Paragastric Cavity. 



Pore. 



