With few exceptions, holothuroids are very slow-moving animals. Many aspidochirotes rear up and extend their anterior ends into the water column when spawning. Other species writhe violently or inflate when they encounter a predator. Some, mostly deep-sea taxa have adaptations for swimming, such as a flattened body or fringes of webbed papillae that can be undulated rhythmically. Epibenthic taxa wander in an apparently random manner as they feed. Many tropical species are nocturnal, living in crevices or under the sand during the day. Others, usually large species, live permanently exposed in shallow water. This lifestyle may be aided by the presence of toxins in the body wall that deter predation by fishes. The juvenile of one aspidochirote species, Pearsonothuria graeffei, appears to mimic the bright coloration of a toxic species of nudibranch gastropod. One species of tiny apodan lives attached to deep-sea fishes. Many species in the aspidochirote Holothuriidae have cuvierian tubules for use in defense. These structures are expelled through the anus, whereupon they expand dramatically in length and become sticky, entangling or deterring would-be predators, such as crabs and gastropods. Disturbance of some holothuroids can cause them to eviscerate. Dendrochirotes eviscerate anteriorly by detaching their tentacle crown. Conversely, many aspidochi-rotes eviscerate through the anus. The eviscerated animals usually live and regrow the expelled organs.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.