Vertebrates vs Invertebrates

Humans have characteristics that can be traced back millions of years to the invertebrates. Invertebrate animals are not primitive and evolutionarily stunted, but rather display adaptations to an amazing variety of environments.

Most animals have the following characteristics:

multicellular,
heterotrophic,
aerobic,
reproduce sexually (and perhaps asexually),
develop from embryos,
and are motile at some point in their life cycle.

Animal life cycles include a period of embryonic development. Three germ tissue layers called ectoderm, endoderm, and mesoderm (present in most species) give rise to adult organs.

Animals with backbones are vertebrates. Animals without a backbone are invertebrates. Body symmetry, degree of cephalization, type of gut, type of body cavity, and presence or absence of segmentation are key characteristics in invertebrate evolution and in distinguishing the different groups.

Body Symmetry and Cephalization

Animals show either radial (round) or bilateral (left and right sides) symmetry. Bilateral animals also show anterior (head end), posterior (tail end), dorsal (back), and ventral (belly) orientations.

As illustrated in this evolutionary lineage, radial and bilateral symmetry led to two major lineages of invertebrates.

Cephalization means having a definite head end, usually with feeding and sensory features.

Type of Gut

The gut is the region where food is digested and then absorbed. Some guts are saclike with one opening for taking in food and expelling waste. "Complete" digestive tracts have two openings (mouth and anus) for continuous food processing, often through specialized regions.

Body Cavities

A coelom (lined with peritoneum) is a space between the gut and body wall that allows internal organs to expand and operate freely. A peritoneum is a smooth transparent membrane that lines the abdomen and doubles back over the surfaces of the internal organs to form a continuous sac.

Some animals (flatworms or Platyhelminthes) are acoelomate. They do not have a coelom but are packed solidly with tissue.

Others, such as roundworms (Nematoda), have a "false" coelom or pseudocoel, not lined with peritoneum.

This animation (no audio) reviews the types of body cavities.

Segmentation

A segmented animal is composed of repeating body units. The units may or may not be similar to one another. Earthworms would be a familiar example in which the segments appear similar. The segments may also be grouped and modified for specialized tasks, as they are in insects.

Evolution of Animals

Animal life originated and primarily evolved in the earth's water provinces, which include lakes, rivers, ponds, estuaries, wetlands, shores, coral reefs, and open oceans.

Animals originated during the Precambrian (1.2 billion - 670 million years ago). One hypothesis is that a ciliate like Paramecium became multinucleate and compartmentalized. Another proposes that multicelled animals arose from colonial flagellated organisms like Volvox.

Perhaps the earliest animals resembled the present-day placozoan called Trichoplax. Two layers of cells make up its flattened body that displays no symmetry, no tissues, and no mouth. Its reproductive modes are as yet unknown.

REVIEW: Major trends in the evolution of animals include
a. cephalization, the development of a definite head region.
b. the development of types of symmetry.
c. variation in coelomic cavities.
d. the development of segments.
e. all of these

REVIEW: A digestive tract is said to be complete if it at least

REVIEW: Which is NOT a general characteristic of the animal kingdom?
a. multicellularity; most have tissues, many form organs
b. exclusive reliance on sexual reproduction
c. motility at some stage of the life cycle
d. embryonic development during the life cycle