often results in problems. Finally, they must reproduce. Reproduction can occur either sexually or asexually, but in one form or another, it has to occur. The ways that these problems are solved varies from organism to organism, and the sum of such "solutions" is unique to each organism. Those solutions define and describe the organisms, but more importantly for hobbyists, they provide a blueprint for the husbandry of those animals. The First Filters B y using " molecular clocks ," it has been possible to roughly deduce when the first organisms that we might call animals appeared in the evolutionary history of life on Earth. This rather momentous event probably occurred sometime between 800 and 1000 million years ago. These were small organisms, and left no fossils, so any discussion of their morphology is largely speculation. However, one thing is clear: they were unable to make their own food by photosynthesis. This lack of photosynthesis is the primary hallmark of being an animal. To obtain nutrition the first animals had to eat other organisms, such as bacteria, or to eat the byproducts of other organisms, such as sugars, mucus or their corpses. Other organisms predated animals, of course, and the most highly developed of these were likely the animal-like protozoans. Although the name "protozoa" conjures up images of primitive "animal" life, in fact, these organisms, such as amoebas , ciliates and flagellates , represent a diverse array of decidedly non-animal life. Presently, animals are defined as multicellular; that is to