In this introduction to the animals, we’ve seen that creatures as different as humans and earthworms have important features in common. At the same time, we saw that even simple animals are remarkably diverse in structure and lifestyle. Animals are the third kingdom of multicellular eukaryotes. They’re heterotrophs that usually ingest their food. Animals have distinctive developmental patterns. Their cells are highly specialized and organized.

Sponges are simple filter feeders whose collar cells resemble choanoflagellates. Most animals besides sponges have symmetrical body plans. Cnidarians and ctenophores, the radiate animals, have radial symmetry. Animals with bilateral symmetry are characterized by cephalization—they have a head or the makings of a head. Bilateral animals can be distinguished by the characteristics of their body cavities: the digestive tract and the coelom. Flatworms are acoelomate. Roundworms and rotifers are pseudocoelomate.

The coelomate animals can be divided into two large groups with distinctive developmental patterns: the protostomes and the deuterostomes. The annelids, a group of segmented worms including earthworms, are an important group of protostomes. This cladogram illustrates some of the probable relationships among the groups of animals we’ve discussed so far, including the deuterostomes, the branch containing humans.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education