Key features of the animal kingdom  (Page 3/20)

Asexual reproduction

Asexual reproduction, unlike sexual reproduction, produces offspring genetically identical to each other and to the parent. A number of animal species—especially those without backbones, but even some fish, amphibians, and reptiles—are capable of asexual reproduction. Asexual reproduction, except for occasional identical twinning, is absent in birds and mammals. The most common forms of asexual reproduction for stationary aquatic animals include budding and fragmentation, in which part of a parent individual can separate and grow into a new individual. In contrast, a form of asexual reproduction found in certain invertebrates and rare vertebrates is called parthenogenesis (or “virgin beginning”), in which unfertilized eggs develop into new offspring.

Classification features of animals

Animals are classified according to morphological and developmental characteristics, such as a body plan. With the exception of sponges, the animal body plan is symmetrical. This means that their distribution of body parts is balanced along an axis. Additional characteristics that contribute to animal classification include the number of tissue layers formed during development, the presence or absence of an internal body cavity, and other features of embryological development.

Art connection

Which of the following statements is false?

1. Eumetazoa have specialized tissues and Parazoa do not.
2. Both acoelomates and pseudocoelomates have a body cavity.
3. Chordates are more closely related to echinoderms than to rotifers according to the figure.
4. Some animals have radial symmetry, and some animals have bilateral symmetry.

Body symmetry

Animals may be asymmetrical, radial, or bilateral in form ( [link] ). Asymmetrical animals are animals with no pattern or symmetry; an example of an asymmetrical animal is a sponge ( [link] a ). An organism with radial symmetry    ( [link] b ) has a longitudinal (up-and-down) orientation: Any plane cut along this up–down axis produces roughly mirror-image halves. An example of an organism with radial symmetry is a sea anemone.

Bilateral symmetry is illustrated in [link] c using a goat. The goat also has upper and lower sides to it, but they are not symmetrical. A vertical plane cut from front to back separates the animal into roughly mirror-image right and left sides. Animals with bilateral symmetry also have a “head” and “tail” (anterior versus posterior) and a back and underside (dorsal versus ventral).