RECALL:
| Protostomes |
Deuterostomes |
| Blastopore --> mouth | Blastopore --> anus |
| Spiral plane of cell division | Radial plane of cell division |
| Determinate cell division | Indeterminate cell division |
| Annelids, Platyhelminths, Molluscs, etc. | Echinoderms, Chordates |
Word Origin:
Echinoderm: echinos = sea urchin (Greek); derma = skin (Greek) [spiny skin]
Major Characteristics of This Group
I. Special structures: Echinoderms are characterized by the presence of tube feet, bulbed structures belonging to a unique water vascular system that develops from part of the larval coelom. Tube feet serve as locomotion, food handling, and respiration structures. They also have pedicellariae, which they use for cleaning, food gathering, etc.
Interestingly, adult echinoderms lack heads, brains, and segmentation.
All echinoderms possess an endoskeleton.
II. Number of Species: There are about 6,500 species of extant echinoderms - all marine (most intertidal or subtidal, and a few dwell in deep-ocean trenches). Thousands of extinct fossil species are known, comprising over 20 classes (compared with 6 today).
III. Symmetry: All Echinoderms have pentaradial symmetry - in which the adult body can be divided into 5 parts around a central axis. Surfaces are therefore given as oral or aboral poles. Radial symmetry derives secondarily from bilateral symmetry of larva, so echinoderms are still in branch Bilateria. Echindoerms are the only eucoelomate animals that are not bilaterally symmetrical as adults.
IV. Feeding Types: Echinoderms exhibit suspension feeding, predation, grazing, scavenging, and deposit feeding. In some species, digestive system is complete (w/ seperate mouth and anus)
V. Respiration: There is no special respiratory fluid; "blood" is primarily coelomic fluid containing amoebocytes that engulf foreign particles
VI. Nervous system: Nervous system consists of a nerve net. Most species do not show any cephalization as adults. Most echinoderms have extensive regeneration abilities.
VII. Reproduction: Sexual reproduction is external. Sexes are seperate. Most echinoderms are oviparous (young hatch from eggs laid outside mother's body.
Development includes passage through various larval stages
Echinoderm Classes
1. Class Asteroidea = sea stars (there is no such thing as a starfish!)
These animals are the classic echinoderms, with five arms
radiating from a central disc. They are active predators and feed by surrounding
their prey and everting their stomachs outside of their bodies and into
the unsuspecting prey item. Their favorite prey, often molluscs, are digested
inside their protective shells!
This is a close-up view of the oral side of a sea star, showing the mouth surrounded by sharp spines. Tube feet are retracted inside the grooves you see radiating out from the center.
2. Class Ophiuroidea (basket and brittle stars) [ophi = greek for snake/serpent]
Brittle stars are agile animals, often using their delicate
arms for swimming. They are easily mistaken for sea stars, but members of
this class have a much smaller central disc and longer, more agile arms.
They are also smaller, on average, than members of class asteroidea. Basket
stars are also members of this class.
3. Class Echinoidea (sea urchins, heart urchins, sand dollars)
These animals are quite different than the brittle stars or sea stars. Sea urchins do not immediately appear to be pentaradial, but once they die and their spines fall off, their test reveals the truth. Sea urchins are active predators of marine plants, and destroyed kelp forests along the Pacific coast when the sea otters (one of the urchin's main predators) were being slaughtered for their fur. Sand dollars are filter-feeders, and will line up in uniform patterns on sandy substrates facing the current to feed.
This is the internal structure of a sea urchin. The right side shows the view from the inside looking down at Aristotle's Lantern (the jaws or teeth of this animal). Material in orange represents the complete digestive system, which is full of commensal ciliate symbionts. The water vascular system is shown in aqua, allowing you to see the true pentaradial pattern. Gonads are shown in red.
4. Class Holothuroidea (sea cucumbers)
Sea cucumbers appear bilateral, but are actually pentaradial - though you have to take a cross section of their body to see it. Cucumbers are sluggish animals and their major defense seems to be that they just don't taste very good - though they are often eaten in sushi. Under stress, cucumbers can eviscerate their digestive organs. Many cucumbers are filter feeders, and other are deposit feeders. Unlike stars, cucumbers have a complete digestive tract.
5. Class Crinoidea (feather stars and sea lilies) [crino = greek for lily].
6. Class Concentricycloidea (sea daisies)
Discovered in 1986. These are deep-sea critters about 1cm across that live on decomposing wood on the ocean floor.
