70 



INVERTEBRA TE MORPHOLOG T. 



water passes into the central cavity, the ccelenteron, 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 nagelluin and 

 resembling greatly Autofiagellata belonging to the genus 

 Codosiga. These cells constitute the endoderni, and between 



Os 



F IG 34. AN ASCON FIG. 35. DIAGRAM TO SHOW THE GENERAL 

 SPONGE, Asceitapri- STRUCTURE OF A SYCON SPONGE. 



mordiaiis (after HAE- The upper portion represents the simplest con- 



cKELfrom SOLLAS). ditiou, the complexity increasing downwards. 



cc = ciliated chamber. ic iuhalent canal. 



Os = osculum. p = iuhaleut pore. 



pr prosopyle 



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

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

 cellular layer. 



In such simple Sponges the mesoglcea is comparatively 

 thin and the pores open almost directly into the creleuteron 

 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 

 c}"linder being, as it were, drawn out into a number of finger- 

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



