The echinoderm (as Webster points out in the node Echinodermata
) is a marine animal
. The name itself comes from Greek 'echino
' meaning spike
' meaning skin
(compare to the Greek for hedgehog
The phylum is divided up into several classes:
The sea stars (star fish) - a central disk attached to some number of arms (typically five, though some branches of this class have more such as the order Velatida (though not all families of this order do) and the order Brisingida). These are slow moving predators that have the ability to regenerate an entire new body from a single arm. One example of this class can be found at Crown-Of-Thorns starfish.
The sea lillys and feather stars - an ancient order (as is all of the phylum) that is well preserved in the fossil record (more than 5000 fossil species have been identified). In ancient times these species were stalked - looking like flowers on the sea floor with a holdfast at the bottom. Today, most crinoids are unstalked and only have the holdfast. On the top of the crinoid are several arms with branches on them. Along the arm and the branches, small tubed feet run and pass food (from small plankton to animal larvae and organic debris) down to the mouth.
The fossil record is abundant with these crinoids - the Mississippian period (fossils found along the Mississippi) are often several feet thick and composed almost entirely of crinoid skeletons.
The "sea daisies" where only discovered in 1986 and have posed a bit of difficulty to the taxonomy of the phylum. This class contains a single genus (the Xyloplax) and two species (one Pacific, one Atlantic). It is possible that these may be related to the asteroids it appears they are far enough separated to remain their own class.
The echinoids are a diverse branch of the echinoderm phylum. This class always has a fivefold symmetry and can be thought of what you get if you take a star fish and roll its arms up until they touch at a point above the mouth. This includes sea urchins and sand dollars.
Unlike other echinoderms, the echinoids always have a set of rigid plates that form rigid skeleton (compared to the flexible starfish and the near skeltonless sea cucumbers). From these rigid plates, spines grow. The spines may be long and pointy as in a sea urchin or short and fuzzy as in a sand dollar. The mouth is composed of five hard teeth in a circle known as Aristotle's lantern.
The sea cucumbers are indeed a member of the echinoderm phyla though the resemblance is not always recognized. These animals also have a five fold symmetry with rows of feet running down the length of the body. These animals live along the floor of the sea (both shallow and deep) and eat sand - or rather the microscopic animals found in the sand. The sand is consumed and passed through the digestive tract. Some estimates state that almost every grain of sand on a beach or sea floor has at one time or another been through this digestive tract since the phylum appeared in the Silurian period (400 million years ago).
This class contains the brittlestars and basketstars. These animals look like delicate five armed starfish though they are not true starfish. The arms are long and delicate (though flexible) and connect to a central disk. The basket stars are larger and have arms that fork and branch. These animals are scavengers though they also eat small crustaceans and worms. Some species filter feed by stretching their arms up and snaring passing plankton.
Common to all the echinoderms, are the following properties:
- A calcite skeleton. While this is not always apparent in animals such as the sea cucumber, it is still there (though microscopic).
- Water vascular system. Water is the basis for all of the echinoderms. This system also makes the fluid filled feet work by pumping water into and out of them.
- Mutable collagenous tissue. The skeleton exists within a tissue that enables it to lock or unlock as whole. This enables the animal to hold a particular position without expending any energy - very useful when trying to open a clam or other prey.
- Fivefold symmetry. While some animals have an irregular symmetry or a higher order (sevenfold or ninefold or more) symmetry, the fivefold symmetry remains as the basis for the structure of the animal.