recap

30.1 recap

We generally recognize animals as multicellular heterotrophs with internal digestion, muscular systems that allow movement, and nervous systems. Animals are thought to be monophyletic because they share several derived traits, especially among their gene sequences. Major developmental differences also provide evidence of their evolutionary relationships.

learning outcomes

You should be able to:

  • Explain the ways that animals differ from each of the other major groups.

  • Contrast the major morphological and developmental features that distinguish major groups of animals.

Question 1

What general features distinguish animals from other major groups of living organisms?

Most animals are multicellular heterotrophs with internal digestion, muscular systems that allow movement, and nervous systems. This combination of features generally allows us to distinguish animals from other groups, although none of these features is diagnostic (by itself) for all animals. Other groups (such as protists, fungi, and plants) include multicellular species; many protists and fungi are heterotrophs; some protists have internal digestion; and not all animals have muscular systems and nervous systems. Evidence for the monophyly of animals comes from gene sequences, as well as a few microstructural features: a common set of extracellular matrix molecules, including collagen and proteoglycans; and unique types of junctions between cells (tight junctions, desmosomes, and gap junctions).

Question 2

Differentiate among the members of each of the following sets of related terms:

  1. radial symmetry/bilateral symmetry

  2. protostome/deuterostome

  3. diploblastic/triploblastic

  1. In radial symmetry, body parts are symmetrical across multiple planes that run through a single axis at the body’s center. Animals with radial symmetry have no front or rear ends, and they are often sessile or drift freely with currents. If they move under their own power, they can typically move slowly equally well in any direction. In contrast, bilaterally symmetrical animals have mirror-image right and left halves divided by a single plane that runs along an anterior–posterior midline. They have front ends that usually contain a concentration of sensory systems and nervous tissues in a distinct head. Bilaterally symmetrical animals usually move forward in the direction of the head, so that the head encounters new environments first.
  2. Among the bilaterian animals, there are two distinct forms of gastrulation—the initial indentation of a hollow sphere of cells early in development that forms the blastopore. In protostomes the blastopore eventually develops into the mouth of the animal, whereas in deuterostomes the blastopore becomes the anus.
  3. Diploblastic animals have embryos with two cell layers (an outer ectoderm and an inner endoderm). The embryos of triploblastic animals have an additional cell layer between the ectoderm and the endoderm, known as mesoderm.

We will begin our exploration of animal diversity by discussing the general features of animal body plans. Later in this chapter we will describe several groups of animals that diverged before the origin of the bilaterians. We will devote Chapter 31 to the protostomes and Chapter 32 to the deuterostomes.