leucones

from The Century Dictionary.

• A group of the chalk-sponges, or Calcispongiæ, characterized first by great thickening of the ectodermal syncytium, so that the inhalent pores, such as exist in Ascones, lengthen into canals which may variously branch and anastomose, and secondly by final restriction to these canals of the endodermal cells, which at first form a continuous layer.