CHAPTER 3 – INVERTEBRATES

Publisher Summary This chapter introduces the biology and medicine of captive invertebrates. Invertebrates represent the largest group of animals on Earth, with approximately 1 million species characterized to date. The only unifying feature common to all invertebrates is the absence of a vertebral column. There are approximately 10,000 different species of cnidarians, and the majority of these animals are found in the marine environment. Cnidarians are classified either as polyps or medusae. Gastropods are primarily aquatic, although terrestrial forms do exist. Among the aquatic gastropods, the majority of animals are found in benthic habitats. There are approximately 70,000 different species of arachnids, and more than 80% of the animals in this group are spiders and mites. The majority of arachnids are terrestrial and carnivorous. Centipedes are from the order Chilopoda, and most of these animals are nocturnal predators. These invertebrates possess fangs, which they use to envenomate their prey or potential predators. The insects represent the largest group of invertebrates, with over 900,000 described species. It is estimated that more than 75% of the animal species on Earth are insects. The greatest diversity among any group of living animals is seen with the insects. Most are terrestrial, but some have also developed the ability to fly. Some species of insects have developed parasitic life cycles, many of which include human involvement.