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Section 29-1: Invertebrate Evolution
As animals became larger and more complex, specialized cells joined together to form tissues, organs, and organ systems that work together to carry out complex functions.
All invertebrates except sponges exhibit some type of body symmetry—either radial symmetry or bilateral symmetry.
Invertebrates with cephalization can respond to the environment more quickly and in more sophisticated ways than can simpler invertebrates.
Most invertebrates with bilateral symmetry also have segmented bodies. Over the course of evolution, different segments have often become specialized for specific functions.
Most animal phyla have a true coelom that is lined completely with mesoderm.
Worms, arthropods, and mollusks are protostomes, and echinoderms are deuterostomes.
Section 29-2: Form and Function in Invertebrates
The simplest animals break down food primarily through intracellular digestion, whereas more complex animals use extracellular digestion.
Respiratory organs have large surface areas that are in contact with the air or water. In order for diffusion to occur, these respiratory surfaces must be kept moist.
Most complex animals move fluid through their bodies using one or more hearts and an open or closed circulatory system.
Most animals have an excretory system that rids the body of metabolic wastes and controls the amount of water in their tissues.
Invertebrates show three trends in the evolution of the nervous system: centralization, cephalization, and specialization.
Invertebrates have one of three main kinds of skeletal systems: hydrostatic skeletons, exoskeletons, and endoskeletons.
Most invertebrates reproduce sexually during at least part of their life cycle. Depending on environmental conditions, however, many invertebrates may also reproduce asexually.