What do we mean by advanced? In this case we mean those invertebrates which are most like animals with backbones are not very efficient living creatures, capable of great variety and different ways of life.
The group of sea-dwelling invertebrates called the Echino-dermata is the most advanced in our sense of the word. Echinodermata means spiny skinned. The word refers to the spines which protect these animals and on which they move.
There are two main divisions of the Echinodermata. Both these divisions have modern, living representatives. There are the free-living forms (meaning that they are not doomed to stay in one place) which include the starfish, sea cucumbers, and the sea urchins.
There are also the forms which are attached in some way to the s ea floor, with names like crinoids and blasted. All the members of the Echinodermata have skeletons of calcite buried within the soft tissue, although sometimes these skeletons are rather reduced in size.
Many invertebrate shells are used for protection. Because the skeletons of this group are internal, they act as a support for the body like our own framework of bones. This internal skeleton can become fossilized and is of interest to geologists.
The two groups which are of most importance in the fossil record are the sea urchins or echinoids, and the crinoids or sea lilies. It is worth nothing here that sea lilies are the common name given to these crinoids. It is an attempt to describe their appearance, and you must remember that they are animals, not plants.
The skeletons of sea lilies are made up of a large number of separate units called icicles. When the animals die these ossicles often become separated so you would be quite fortunate to find a whole fossil crinoid. You are more likely to find the individual parts, but even these tiny fragments can be positively identified.
The skeleton or test of the echinoid is made of plates of calcite and the animals grow by adding more calcite to each plate. Spines also develop by accumulation of calcite. This skeleton is very strong because of the way in which the plates are arranged, and whole fossils may be found, for example, in flints in the chalk. Echinoids first appeared about 500 million years ago during a period which we refer to as the Ordovician, and there are still sea urchins in modern oceans.
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