Introduction to Animal Morphology. 73 



The plastides are, in simple Sponges, arranged in 

 two layers, forming an outer (ectodermal) and an 

 inner (endodermal) wall for the central cavity. They 

 may be cytodes or cells, often amoeboid and naked, or 

 with a thin, rarely a thick, cell wall ; some or all of 

 them being ciliated or flagellate. In some forms the 

 protoplasm is undifferentiated, with free nuclei. The 

 interstitial pores often are inconstant, but may become 

 branching varicose canals. The walls of the central 

 -cavity frequently become irregularly folded, and by 

 coalescence of the several folds and dilatation of their 

 communicating pores, the single central cavity be- 

 comes the focus from which irregular tubular branches 

 -s; by lateral budding, new zooids form beside the 

 older ones, and the branches from the central cavities 

 of each, communicate with the irregular sinuous inter- 

 zooidal spaces, thus giving rise to the complicated 

 svstem of passages found in such colonies as those of 

 the common toilet Sponge. In some the pores enlarge 

 into pseudoscula (Geodiinae), or even into true oscula 

 -^ f ac lay]. 



es, as in Axinella, and less distinctly in 

 Raphiophora, there is a regularly radiant system of 



tls from the osculum, foreshadowing the radial 

 symmetry of Coelenterates. 



Through the surface pores currents of iluid enter 

 th- body cavity, propelled inwards by th' cilia or 

 Ha of tin- lining plastiilcs,* and rxpdlrd through 

 tin? larLT*' osrulum or <-ifTcnl api-rUm'. I li> lining 

 s an- nourish,-.! by tin- organic particles in tin' 



uniformly distributed, <>r ,-Kr then- m.iy l>e here and 

 there sm.t'' while the intervenin.' 



