498 



A MONOGRAPH OF THE AUSTRALIAN SPONGES, 



intermediate varieties are unchanged descendants of certain stages 

 of a changing series of generations which finally formed the most 

 differentiated species, we will have to consider the table as a true 

 ancestral tree. For example : — 



If b are unchanged descendants of B, and B is one of the real 

 ancestors of the species a being formed, b can be considered as an 

 ancestor. 



In the same manner d is the unchanged descendant of D. D 

 a real ancestor of e and f, and therefore d can be considered as a 

 representative of the real ancestor. 



I insert this self-evident explanation here to avoid the possi- 

 bility of my ancestral tables being misunderstood. 



Histology. 



A muscular membrane extending in a tangental direction below 

 the sub-dermal cavities has been observed in this species. This 

 membrane is throughout i)arallel to the outer surface, and composed 

 of the ordinary spindle-shaped muscular cells which form several 

 layers in it. At the conuli it rises up to the surface, and there it 

 joins the muscular system of the outer skin. This skin is per- 

 forated by the canals which lead from the sub-dermal cavities into 



