Echinoids

Echinodermata is a phylum of spiny, invertebrate marine animals, known as echinoderms.
They are characterised by an internal skeleton of porous calcite plates, 5-fold symmetry and an internal water vascular system, all of which are described below).
The Echinodermata phylum contains a number of classes, some living, some now extinct. The living classes include Ophiuroides (brittle stars), Asteroidea (starfish), Echinoidea (sea urchins), Holothuroidea (sea cucumbers) and Crinoidea (sea lilies).

The following discussion covers:

• Echinoidea class
• Skelton morphology
• Spines
• Body cavity
• Regular and Irregular forms
• Regular Echinoids
• Irregular Echinoids

Echinoidea Class

The Echinoidea class contains the echinoids, which contain a variety of [orders?] including sea urchins, sand dollars and heart urchins. They are free-living animals, whose body is enclosed in a globular, sometime heart-shaped test, built of sections of interlocking calcitic plates.

Because many member of the class survive today, reconstruction of the form of soft parts of now-extinct species found only as fossils can be made fairly accurately, using the "Nearest Living Relative" (NLR) technique.

It is particularly important in the study of Chalk, as it is one of the most common fossils found there. Micraster is particularly closely associated with the the Upper Cretaceous Chalk.

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Skeleton Morphology

There are 2 features that are shared by practically all echinoids, being

• A five-fold symmetry (modified to 2-fold in some forms), and
• A skeleton composed of tiny individual plates which are closely joined, giving the superficial appearance of a single unit.

Each of the 5 sections of the skeleton consists of an ambulacrum, and an interambulacrum - so the whole structure consists of 5 pairs of alternating zones, ambulacra and interambulacra (like an orange divided into 5 segments, each of 2 parts).
[insert sketch or reword or both]
Near its top surface is its anus, and near the bottom are its mouth and 'tubular feet'.

A hollow skeleton such as this, composed of many interlocking pieces, is known as a test. The tests of echinoids is made of calcite, which is the more stable form of calcium carbonate. Fossil echinoids are thus more commonly found than fossil bivalves (with shells of aragonite).

The 'mode of secretion' of calcium carbonate in an echinoid is such that the plates are secreted within a thin layer of soft tissue. This makes the echinoid skeleton internal, so it is in fact an endoskeleton - even though the organs are inside the skeletal framework. [is this correct?]

Each echinoid plate is perforated with fine pores, so under a microscope, the skeleton resembles a sponge-like 3-dimensional meshwork. In life, this meshwork is mostly filled with a fibrous protein, callogen, so that callogen fibres bind the plates together in life. On death, the skeleton either collapses as the callogen decomposes, or retains its shape by post-[death?] mineralisation of calcite, which may fuse the plate together.

A curious characteristic of echinoderm plates is that each plate is a single crystal, which is convenient for identification in thin section.

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Spines

Echinoids are covered in spines, attached to the (visible) holes (the tubercles) of the outer skeletal surface.

In living echinoids, the spines act as defence, and in some species, are poisonous. They are also used in locomotion, to 'lever' the animal across the sea floor. (In this way, they may perform a similar function to that of the 'tube feet', varying between different species).
In fossilised echinoids, the spines are often found close by, isolated in the sediment. Only in exceptional circumstances are they found still attached to the shell. Spines are attached to the shell by 'ball and socket' joints and are controlled by muscles. This may have been for defence - stiffening the spine when touched, or swivelling toward a disturbance - or to aid with locomotion. As these muscles rot after death, the spines tend to fall off.

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Body Cavity

The inside of an echinoid is unique. It contains arrays of hydraulically-operated tentacles, connected to a system of fluid-filled canals, known as the 'water vascular system'. A ring-shaped canal surrounds the gut, and has branches to each of the 5 ambulacra. Additionally, a single tube from the ring canal leads to a special perforated skeletal plate. This connection of the internal system to the outside allows the pressure between the animal's internal water system, and the surrounding seawater, to remain in equilibrium.
The 5 branching tubes each connect in many places to the pores of the ambulacra, where they [protrude?] as 'tube feet'. (Each tube foot [leaves?] the ambulacrum by a pair of pores, so there are twice as many pores are there are tube feet).
These tube feet may be used for locomotion, or to assist feeding, by "sweeping" food towards the mouth. They appear to have a similar function to the spines, depending on species specialisations. [maybe!]

The remainder of the interior of the echinoid is divided by thin membranes into a series of fluid-filled cavities. Small cavities surround the anus and mouth. A main large cavity has 5 reproductive bodies suspended in it. These produce eggs and sperm, which are released from 2 pores in the 'genital plates' of the ambulacra. There may be between 1 and 5 such genital plates, depending on species.

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Regular and Irregular forms

Echinoids can be classified into 2 forms, regular and irregular. The regular echinoids are the earlier ones, when they showed 5-fold symmetry. These creatures were epifaunal - i.e. lived and moved on and over the sea floor. Echinoids evolved with time, and developed a more irregular morphology, that began to gain a bilateral (2-fold) symmetry in place of the early 5-fold.
This change came about as the echinoids began to burrow into the soft sediments, and became burrow-dwelling (infaunal) in their living habits. Consistent movement in one direction, along a burrow (rather than free-moving across the sea floor) is more efficient with a 2-fold symmetry, which was thus an evolutionary development following the change in living habit.

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Regular Echinoids

A regular echinoids, such as .

Note from the sketch that the mouth of the echinoid is on the underside of the creatures, along with its feet, whilst its anus is at the top - to allow waste to be rapidly removed by water movement, away from the area of feeding. Specialise forms of tube feet were used for both locomotion and feeding. In life, the oral [mouth] surface was covered in a flexible membrane studded with plates, with the mouth in its centre. This mouth contained a complex structure, that included 5 teeth, operated by muscles.

This feeding apparatus was possibly used to rasp (or scrape) algae off from rocky surfaces, or sessile prey such as coral polyps. These early echinoids had thicker spines near the top, and smaller ones near their mouths (to prevent interruption during feeding).

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Irregular Echinoids

With their evolution from epifaunal to infaunal living habits, the body form of the echinoid changed to become irregular. Micraster has a mouth and anus in very different positions from that of the regular echinoid, due to the evolutionary forward movement of the mouth, and backward movement of the anus, from their original central positions in earlier forms. These positions are correlated with feeding and waste disposal in a burrow, Micraster was probably a deposit-feeder, with a lower lip equipped with spines against which specialised feeding 'tube feet' scraped food.

Micraster had its largest spines on its bottom surface (near the mouth), probably for locomotion, whereas the rest of its spines were much shorter and finer, almost like a hair coating of the animal.

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For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk