IT is not unusual to find two different sponge species growing together intimately. One sponge often uses another as substrate, either encrusting, growing up from it, or surrounding it laterally. Topsent1 and Kirkpatrick2 have each reported a case in which one sponge is completely surrounded by another, with only its osculae projecting to the exterior. There is, however, always a definite delineation between the two unless the substrate portion of the host is dead.