Starfish, sea cucumbers (holothurians), sea urchins and brittle stars are examples of echinoderms.
Echinoderms are marine animals. They live in salt water.
Echinoderms, as their name indicates (echino = spiny, derma = skin), are creatures with spines that stick out from an endoskeleton. Their endoskeleton is made of calcareous plaques that, in addition to spines, contain pedicellaria, small pincers used to clean the body and to help capture prey. They also contain a hydrovascular system known as the ambulacral system. Adult echinoderms have pentaradial symmetry; the radial symmetry in these animals is secondary, as it is present only in adults.
The skeleton of echinoderm is internal; that is, it is an endoskeleton. It is made of calcium carbonate (calcareous).
Vertebrates also have an internal skeleton made of bones and cartilage. Arthropods have an external shell made of chitin, a chitinous exoskeleton. Some molluscs have a calcareous shell that functions as an exoskeleton.
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The system that allows echinoderms to move and to attach to substrates is called the ambulacral system. In these animals, water enters through a structure called the madreporite, passes through channels and reaches the ambulacral feet along the undersurface of the body. In the ambulacral region in contact with the substrate, there are tube feet which empty and fill with water, thus acting as suckers.
Echinoderms contain a complete digestive system, with a mouth and anus.
Sea urchins have a teeth-like structure attached to the mouth and made of five teeth connected to ossicles and muscle fibers. This structure, known as Aristotle’s lantern, is use to scratch food, mainly algae, from marine rocks.
Echinoderms and chordates are deuterostomes, meaning that, during their embryonic development, the blastopore turns into their anus. All other animals with complete digestive system are protostomes, meaning that their blastopore turns into their mouth.
The blastopore is the first opening of the digestive tract to appear during embryonic development.
Phylum Echinodermata Review - Image Diversity: blastopore
In echinoderms, respiratory and circulatory systems are not well-defined (with the exception of the holothurian group). The ambulacral hydrovascular system carries out the tasks of these systems.
Echinoderms do not have an excretory system. Their excretions are eliminated by diffusion.
Adult echinoderms, along with cnidarians, present radial symmetry, meaning that their body structures are distributed around a central point. However, the radial symmetry in echinoderms is secondary radial symmetry, since their larval stage has bilateral symmetry and the radial pattern appears only in adult specimens (there are a few adult echinoderms with lateral symmetry). All other animals have lateral symmetry with the exception of poriferans (they have no defined symmetry).
Echinoderms do not present cephalization. They have a diffuse network of nerves and neurons made of a neural ring around the mouth and radial nerves that split off into branches to follow the pentaradial structure of the body.
Fertilization among echinoderms is external, as gametes are released into the water, where fertillization occurs.
The majority of echinoderms are dioecious, containing both males and females.
In echinoderms, embryonic development is indirect, with ciliated larvae.
The five classes of echinoderms are: asteroids (starfish), ophiuroids, crinoids, holothuroids (sea cucumbers) and echinoids (sea urchins and sand dollars).
Examples of representative species: sea cucumbers, sea urchins, starfish. Basic morphology: calcareous endoskeleton with spines, ambulacral system. Type of symmetry: secondary radial. Germ layers and coelom: triploblastics, coelomates. Digestive system: complete, deuterostomes. Respiratory system: nonexistent. Circulatory system: nonexistent. Excretory system: nonexistent. Nervous system: simple, nerve network without ganglia or cephalization. Type of reproduction: sexual, with a larval stage.
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