say, built of many smaller component parts called "cells." Protozoans lack cells. Although such organisms may be thought of as having but one cell, it probably is better to consider them as lacking cells altogether. Many protozoans, for example, have multiple copies of internal cellular structures; they just seem to lack the internal division into cells. Good aquarium examples of such intracellular structural multiplicity may be found in Caulerpa and other similar algae. Each individual algal "plant" contains several thousand nuclei , but these nuclei are not found within individual cells. Consequently, each individual of Caulerpa may be considered to be either a single-celled organism with many nuclei, or a multinucleate organism without cells. Some protozoans may be "colonial" organisms, having several almost independent "individuals" attached to a single stalk or growing from a common base. Colonial organisms are particularly common within the two groups of protozoans referred to as the ciliates and the flagellates . It is within the flagellates that are found the organisms that appear to be most similar to what is likely the ancestor of sponges and, by inference, the ancestor of all animals. These organisms are called choanoflagellates ( 2 , 3 , 4 , 5 ). Colonial choanoflagellates are clusters of small, rather spherical cells, each of which has a structure resembling the top flaring part of a funnel on its surface. In the center of this funnel is a single microscopic hair-like flagellum. As the flagellum beats it creates minute water currents that