Litroduction 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 

 pass ; 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 

 system of passages found in such colonies as those of 

 the common toilet Sponge. In some the pores enlarge 

 into pseudoscula (Geodiinee), or even into true oscula 

 [MiklucJw-J\Iaclay) . 



Sometimes, as in Axinella, and less distinctly in 

 Raphiophora, there is a regularly radiant system of 

 canals from the osculum, foreshadowing the radial 

 symmetry of Coelenterates. 



Through the surface pores currents of fluid enter 

 the body cavity, propelled inwards by the cilia or 

 flagella of the lining plastides,* and expelled through 

 the large osculum or efferent aperture. The lining 

 plastides are nourished by the organic particles in the 



* These may be uniformly distributed, or else there may be here and 

 there small cavities lined by ciliated cells, while the intei-vening passages 

 possess only amcebiform plastides. 



