636
How do we recognize an organism as an animal? That may seem obvious for many familiar animals, but less so for groups such as sponges, which were once thought to be plants.
focus your learning
Most animals are multicellular heterotrophs with internal digestion, muscular systems that allow movement, and nervous systems, although none of these features is completely diagnostic for all animals.
A few microstructural features and genome sequences provide the best evidence of animal monophyly.
Several patterns of embryonic cleavage exist among animals.
Distinct layers of cells form during the early development of most animals.
Bilaterians, named for their pattern of symmetry, include protostomes and deuterostomes.
Some of the general characteristics we associate with animals include:
Multicellularity. In contrast to the bacteria, archaea, and most protists (see Chapters 25 and 26), all animals are multicellular. Animal life cycles feature complex patterns of development from a single-
Heterotrophic metabolism. In contrast to most plants, all animals are heterotrophs. Animals are able to synthesize very few organic molecules from inorganic chemicals, so they must obtain the necessary organic molecules from their environment.
Internal digestion. Although the fungi are also heterotrophs (see Chapter 29), animals and fungi digest their food differently. While fungi digest food outside their bodies, most animals use internal processes to break down materials from their environment into the organic molecules they need. Most animals ingest food into an internal gut that is continuous with the outside environment and in which digestion takes place.
Movement and nervous systems. In contrast to the majority of plants and fungi, most animals can move their bodies. This movement is often coordinated through a well-
637
Although these general features help us recognize animals, none is diagnostic for all animals. Some animals do not move, at least during certain life stages, and some plants and fungi do have limited movement. Some animals lack a nervous system, and the diffuse nerve nets (which lack a coordinating brain) of ctenophores and cnidarians are distinct and independently evolved from the central nervous systems of bilaterians. Some animals lack a gut, and many multicellular organisms are not animals. So on what basis do we group all animals together in a single clade?