20 



SPONGES 



dermal layer. The inhalent canals open to the exterior by the ostia, and also 

 communicate with the radiating chambers by means of small apertures, the 

 prosopyles. The radiating chambers open into the gastral cavity through 

 the apopyles. Thus the water enters the ostia or inhalent pores into the 



Middle Jelly 



Dermal Epithelium 



Scleroblasts 



Inhalent Pore 



Porocyte 



Fig, 6.— Transverse Section of an Ascon Sponge (diagrammatic). 

 After J. Graham Kerr. 



incurrent or inhalent canals ; thence it passes through the prosopyles into the 

 radiating chambers or radial canals, and from these it goes through the 

 apopyles into the gastral or paragastric cavity and out by the exhalent 

 osculum. Example : <,%con (Class, Calcarea ; Order, Heterocoela). {See Figs. 4 

 and 7.) 



3. The Leucon Type. 



By a further process of folding this type is derived from the Sycon type, 

 the result being that diverticula of the gastral cavity arise and into each 

 diverticulum several radial canals open. Thus the primary incurrent canal 

 communicates' by means of secondary inhalent canals, from which the 

 prosopyles arise, with a secondary radial canal lined by choanocytes, which, 

 opens by an apopyle into the primary radial canal or radiating chamber. 

 All the primary radial canals open into the gastral cavity. Therefore the 

 water enters through the ostia" into the primary incurrent canals; thence it 



