Invertebrate Animal Diversity: Echinoderms
{see pp. 141 - 145 in Castro & Huber text}
The final group of invertebrate animals we will examine are the echinoderms [e-kine-oderms] (Ph. Echinodermata). Their name translates as “spiny-skinned”, indicative of
the presence of various-sized spines that may be found covering the surface of their
bodies. A more significant trait shared by this group, though, is a return to radial
symmetry that is exhibited by most of the adult animals. The symmetry of echinoderms
is unique in that it is pentamerous, meaning the body plan is based on a set of five
appendages radiating out from a central disc giving many echinoderms a star shaped
body. Another trait unique to echinoderms is the reduction or loss of a typical circulatory
system in favor of a water-vascular system that also functions in locomotion in some
groups. Biologists recognize six classes of echinoderms; we will focus our attention on
four of them.
Part 1 – Sea stars (Cl. Asteroidea) [as-ter-oy-dee-a]
Sea stars are the best examples to use when first learning the basic echinoderm body plan
because they exhibit all of the traits with large, obvious structures. First, the body itself
generally consists of five fleshy, muscular arms that radiate from the center. The aboral
surface (remember, radially symmetrical animals don’t have dorsal/ventral or left/right
sides) is covered in a variety of structures. Most noticeable will be the short spines that
protrude through the skin. These spines are actually extensions of the numerous ossicles
found underneath the skin that serve as the animal’s skeleton. Some of the spines are
modified into pincer-type organs called pedicellariae that are used to remove sediment
and debris from the surface of the animal. There are also soft, sac-like structures called
dermal gills that are located between the spines, and yes, they are used for gas exchange.
On most sea stars there is also a prominent, whitish disk located near the angle of two of
the arms. This structure, called a madreporite, acts as a sieve plate to filter water
entering the water-vascular system. This system circulates sea water into an extensive set
of hydraulically operated tube feet. From the madreporite water flows down a tube
reinforced with calcium carbonate rings called the stone canal into a circular ring canal.
Branching off of the ring canal are the radial canals that extend down the length of each
arm. Each tube foot branches off of a radial canal and consists of a muscular bulb, the
ampulla, that is found inside the body cavity and a tubular podium that extends out of
the body into an ambulacral groove found on the oral surface of each arm.
Locomotion in sea stars involves the use of hydraulic pressure when the muscles of the
ampulla contract. A muscular valve prevents water from flowing back into the radial
canal, forcing it down into the podium causing the foot to stretch out. A suction disk on
the end of the podium is used to attach the foot to the substrate, then the muscles of the
podium contract to pull the foot in a particular direction. The tube feet appear to act
somewhat independent of each other, yet sea stars are capable of purposeful movement.
Go to YouTube.com and search for videos using the terms “Seastar Tube Feet”
to see these organs in action.
The internal structure of a sea star continues to reflect the pentamerous radial symmetry
of the outside. The digestive system occupies most of the central disc with a two part
stomach. The cardiac stomach is found closest to the mouth and is usually everted out
of the mouth to begin the digestion process outside of the body. Food is then drawn up
through the mouth and passed into the pyloric stomach where further digestion occurs.
Digestion is aided with secretions from the digestive glands found in each arm.
Underneath each digestive gland is a pair of gonads.
Using the information above and the figures below, perform a dissection of a
preserved sea star to identify all of the bold faced structures. Start with the
external structures then use a set of scissors to cut off the aboral surface around
the central disc and one of the arms. When removing the central disc cut around
the madreporite so that it remains intact.
Part 2 – Brittle stars and basket stars (Cl. Ophiuroidea) [o-fee-yur-oy-dee-a]
Brittle stars bear a strong resemblance to sea stars since both groups generally have five
arms that radiate from a central disc. Brittle star arms are much more slender than and
not as muscular as sea star arms. They are also proportionally longer, sometimes five to
ten times the length of the central disc. These long, snaky arms are used to pull the
animal along the bottom, so their rate of locomotion is significantly greater than that of
sea stars. The tube feet are strictly used to filter water for food and pass it along to the
mouth. All of the internal organs are restricted to the central disc. A number of brittle
star species have been studied by physicists interested in optics and lenses. Apparently
some of the calcium carbonate crystals that make up the central disc are near perfect
lenses, a discovery that has prompted biologist to speculate that brittle stars use the entire
disc as some sort of eye. Nerve fibers do connect to each of the crystals, but it remains to
be seen if they respond to visual stimulation.
Basket stars have a similar construction to brittle stars, but the arms are extensively
branched and curled. Basket stars are also filter feeders, but they tend to be nocturnal.
Examine the dried specimens of brittle and basket stars on the side counter or
perform an image search for these animals and sketch an example of one in the
space below. Label the central disc and the arms.
Part 3 – Sea urchins and sand dollars (Cl. Echinoidea) [ek-in-oy-dee-a]
The sea urchins are the epitome of their phylum with their long, pronounces spines
radiating from a spherical body. The internal skeleton is the result of fusion of the
calcareous ossicles seen in other echinoderms into a single piece called the test. The
spines pivot against the test via small ball-and-socket joints which gives them a nice
range of motion. Sea urchins can draw the spines in together to ward off the claws of
crabs or spread them apart to allow long-stalked pedicellariae to pinch at the soft parts of
an attacking sea star.
The spines of some species are equipped with venom sacs to better ward off predators.
Also found in between the spines in five pairs of rows are long, slender tube feet that the
animals can use for locomotion or to transfer food items down to the mouth. Most sea
urchins are herbivores, feeding on various species of macroalgae. They utilize a unique
five-piece jaw apparatus called Aristotle’s lantern to chew their food (see fig. 7.48 on p.
143 of the Castro & Huber text). Their digestive system is a long, winding tube, an
adaptation to their herbivorous diet which requires more processing of food.
The tests of sand dollars are familiar to beach combers all over the world, but few people
realize that the live animals are actually covered in a layer of fine spines that give them
an almost fuzzy texture. Sand dollars use these spines to move around their sandy sea
floor habitats and collect small food particles out of the water. Their flattened bodies are
an adaptation to the more turbulent wave action and lack of protective cover in their
environment. The distinct “petal pattern” visible on the aboral surface is produced by
pores where tiny tube feet protrude through the test. Related to the sand dollars are the
heat urchins. These animals have the same small spines and petal pattern of sand dollars,
but have oval, rounded tests like sea urchins.
Examine the dried specimens of sea urchins and sand dollars on the side
counter or perform an image search for these animals and sketch an example of
each in the space below.
Part 4 – Sea Cucumbers (Cl. Holothuroidea) [ho-lo-thur-oy-dee-a]
Sea cucumbers are confused echinoderms. They have lost most of the outward
characteristics of pentamerous radial symmetry, opting instead for a body plan that is
almost bilaterally symmetrical. The body is elongated, tubular and lacking both ossicles
and spines. The tube feet are concentrated in five rows along one side of the body. There
are some species that have five sets of branched tentacles arranged around the mouth that
are used to collect sediment. Without spines for protection, sea cucumbers rely on a thick
rubbery or leathery skin that makes them almost inedible. Some species can also distract
predators by expelling the hind part of their digestive tract in a sticky mass of organs
which will later be regenerated. Sea cucumbers are also odd in that they use a pair of
branched passageways off of the hind gut called respiratory trees for gas exchange.
Water is circulated through these structures by pumping water in and out of the anus.
Perform an image search for “sea cucumbers” and sketch an example in the
space below.
Part 5 – Reproduction in Echinoderms
The general pattern for reproduction in echinoderms is sexual. Nearly all species have
separate males and females that release their gametes into the water (broadcast
spawning). The eggs develop into a variety of larval forms depending on class (see fig.
15.11 b and c p. 340 of the Castro & Huber text for two examples). What is interesting
about these larvae is that they exhibit bilateral symmetry and have to undergo
metamorphosis to become pentamerous adults. It is also interesting to note that some of
the early embryonic stages of echinoderms are similar to those of chordates, the phylum
that includes all vertebrate species.
Lab Summary Questions
1. What are the three traits that would allow you to classify an animal as an
echinoderm? ________________________________________________
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2. What is the general function of the structures called pedicellariae as used by
sea stars? ___________________________________________________
3. How many stomachs does a sea star have and how is each one used? ___
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4. In what ways are brittle stars different from sea stars? __________________
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5. What is a test as it applies to echinoderms? _________________________
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6. In what ways are sea cucumbers not your typical echinoderms? __________
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