70 



INVERTEBRATE MORPHOLOGY. 



water passes into the central cavity, tlie coelenteron, and 

 escapes by the osculum. The exterior of the body is covered 

 by a layer of flat cells, the ectoderm, and the coelenteron is 

 lined by collared cells provided with a single flagellum and 

 resembling greatly Autoflagellata belonging to the genus 

 Godosiga. These cells constitute the endoderm, and between 



OS 



Pia. 34.— An Ascon Fig. 35. —Diagram to show the General 

 SeosGK.Ascetiapri- Structure of a Sycon Sponge. 



mordialis (after Hak- The upper portion represents the simplest con- 



cKELfrom SoLLAs). dition, tlie complexity increasing downwards. 



ec = ciliated chamber. ic = inhalent canal. 



Os = osculum. p = inhalent pore. 



pr — prosopyle, 



it and the ectoderm is the mesogloea, in which are imbedded 

 large numbers of cells, giving it almost the appearance of a 

 cellular layer. 



In such simple Sponges the mesogloea is comparatively 

 thin and the pores open almost directly into the coelenteron 

 lined by the collared cells. This arrangement constitutes the 

 first or Ascon type of structure. In the majority of forms a 

 much greater complexity arises from the walls of the simple 

 cylinder being, as it were, drawn out into a number of finger- 

 like processes, each of which communicates by a wide open- 



