Explore animal body plans, symmetry, arthropod features, and the diversity of animals without backbones.
Part of the free Knowledge Mountain biology library. Read an explanation, try a question, and use a memory cue to check what you can recall.
Invertebrates are animals without a vertebral column. They include sponges, cnidarians, flatworms, mollusks, annelids, arthropods, echinoderms, and many other groups.
Compare groups using inherited traits and body plans, such as tissues, symmetry, segmentation, and developmental patterns. “Invertebrate” describes absence of a backbone rather than one exclusive evolutionary branch.
Which animal group has jointed appendages and an external skeleton that must be molted for growth?
Arthropods.
Insects, spiders, and crustaceans are arthropods. Their exoskeleton provides protection and support, while joints allow movement. Molting permits an increase in body size.
What body symmetry is often associated with a clear head end and movement in a preferred direction?
Bilateral symmetry.
A bilateral body can be divided into left and right mirror halves. Concentration of sensory structures at the front is called cephalization. Radial symmetry suits different body organizations.