Echinoid Morphology

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  • Created by: naomi
  • Created on: 04-01-14 20:47

Echinoids

Phylum: Echinoidermata

Class: Echinoidea

Echinoids are sometimes called sea urchins, and like star fish have five-fold symmetry

They have skeletons or tests mad eup of thin plates covered in spines

Through these paltes tube feet proturde

These are used for ovement across the sea floor and for attaching the animal to rocks or for burrowing, as well as for respiration and feeding.

There are two groups of echinoids with different shapes:

Those with five-fold symmentry are called regular echinoids

Those with bilateral symentry are called irregular echinoids 

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Echinoids

Phylum: Echinoidermata

Class: Echinoidea

Echinoids are sometimes called sea urchins, and like star fish have five-fold symmetry

They have skeletons or tests mad eup of thin plates covered in spines

Through these paltes tube feet proturde

These are used for ovement across the sea floor and for attaching the animal to rocks or for burrowing, as well as for respiration and feeding.

There are two groups of echinoids with different shapes:

Those with five-fold symmentry are called regular echinoids

Those with bilateral symentry are called irregular echinoids 

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The Skeleton

  • The test of a typical echinoid is hemispherical
  • it consists of many interlocking plates arranged in 10 double columnds - 5 ambulacra and 5 larger ambulacra - which radiate from the **** in the centre of the upper surface to the outh in the centre of the lower surface
  • the ouside of the test is covered by skin
  • spines are held in position and moved by muscles attached to the test
  • each plate and spine is made of a single crystal of calcite
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Plates

  • Two rows of ambulacral plates make up one ambulacra
  • the ambulacral plates are small and each is pierced by one pair of pores called a pore pair
  • the tube feet extend out through the pores as they are needed filled with water from the water vasular system
  • the interambulacral plates are large and have no pores
  • their surface is covered by many tubercles to which, in life,  movable spines are attached by muscles
  • two rows of interambulacral plates make up one interambulacra 
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The Apical System

  • the apical system in regular echinoids consists of 10 plates arranged in one or two rings around the periproct which is the membrane around the ****.
  • one genital plate is also the amadeporite, a finely perforated plate whicch allows water to enter the water vasular system
  • in irregular echinoids, the apical system does not enclose the periproct and ****, rather it is small and compact or elongate
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The Mouth

  • the peristome is the membrane which surrounds the mouth on the lower or oral surface
  • regular echinoids have jaws 
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Spines

  • a tubercle consists of a round knob - the mamelon - protruding from a shallow mound - the boss
  • they occur on both ambulacral and interambulacral plates, but are much more common on interambulacra
  • a spine sits on the tubercle
  • in regular echinoids the spines are very variable in size and shape ranging from large, thick, club-like forms to long thin needles
  • in irregular echinoids the spines are more regular in size and close set often like bristly fur
  • spines are used as a means of protection and for walking or burrowing
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Irregular Echinoids

  • irregular echinoids are heart shaped rather than round, making them bilaterally symmetrical
  • the **** is on the reat, or posterior, edge of the skeleton
  • the mouth, although still underneath, is slightly forward of the centre, set in a groove called the anterior groove which allows currents of water, agitated by the tube feet, to bring in food
  • the ambulacral areas may not extend all the way down to the mouth but form a petal-like pattern on the top of the test instead
  • these echinoids live in burrows, unlike the regular echinoids 
  • in the irregular echinoids, in which the mouth lies towards the anterior end, jaws and perignathic girdle are lacking
  • the posterior interabulacrum is extended towards the mouth as a broad ridge - the plastron - its forward end may project as a lip called the labrum on the lower side of the mouth
  • fascioles are relatively smooth bands of ciliated spines on the test of some irregular echinoids - such as the sub anal fasciole. 
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Crinoids

Phylum: Echinodermate

Class: Crinoidea

  • the most commonly fossilised typed of crinoid originally lived tixed to the sea bed
  • they appeared almost plant like and are sometimes referred to as sea lilies
  • there are three main parts:

1.) a flexible stem made of calcite plates called ossicles. The shape of the stem ossicles varies from round to pentagonal to five starred. It may have had an anchoring 'root' but it did not draw up nutrients as a plant would. An axial canal runs up the centre of ossicles

2.) a hollow calyc where the main soft body lives. This was made of calcite plates arranged in multiples of 5. The individual plates are usually hexagonal or pentagonal in shape and are arranged symmetrically in circlets

3.) a system of five main brachia or arms which in many species branched out into smaller structures. These parts of the body had grooves running along them and acted as a kind of unnel lined with tube feet, directing food such as plankton towards the animal's moiuth at the top centre of the calyx. 

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Crinoids

  • on death, the crinoid skeleton is prone to disarticulation and whole skeletons are rare
  • bioclastic limestones composed of crinoid ossicles are very common
  • during the carbonifeous, in particular, crinoids were a very important component of the shallow water ecosystem and grew in large 'thickets' swaying backwards and forwards in the wave-agitated waters
  • they lived by filtering planktonic material from the waters using tube feet on the brachia and passing the food particles back to the mouth 
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Crinoid questions

1.) What part of the crinoid is most commonly found in crinoidal limestone?

Ossicles

2.) Why are the arms rarely preserved?

They are made of hard tissue but are prone to disarticulation

3.) Describe how crinoids feed 

The tube feet catch whatever is going past through filter feeding. The funnel lined with tube feet directed food such as plankton towards the animal's mouth at the top centre of the calyx

4.) Describe the mode of life of stemmed crinoids

They live on the sea floor due to being fixed to the sea bed. They filter feed through tube feet. They are sessile, meaning they are attached to the sea floor. They live with other shallow animals e.g. corals 

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Crinoid questions

5.) Suggest a likely rock in which crinoid fossils may be found

Crinoidal limestone

6.) When are crinoids most common in the geological record?

Carboniferous (Palaeozoic)

7.) What is the difference in mode of life between modern crinoids and most shallow carbonate sea ancient crinoids?

The modern ones are mainly nectonic and sessile ones live much deeper than expeected - the older, more ancient ones lived in shallow seas. 

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Echinoid morphology related to mode of olife

Echinoids are free moving marine invertebrates walking and burrowing on the sea floor down to a depth of about 1000m. 

Echinoid morphology is related to how it lives either grazing and scavenging on the sea floor or filter feeding in a burrow. 

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Movement

Regular echinoids:

  • epifaunal mode of life
  • live mostly on rocky shores and their tests show five fold radial symmentry reflecting their ability to move freely in any direction
  • spines attached to the interambulacral plates at a spine boss by muscles allow the echinoid to move across the sediment
  • each spine rotates freely allowing the animal to propel itself across the sea floor
  • the size of the spine bosses (tubercles) on the interambulacral plates reflect the size and weight of the spines
  • spines are often sharp and brittle and have a protective function as well
  • regular echinoids use tube feet emerging from pores in the ambulacral plates as suckers for clinging to rocks and moving across rocky areas of the sea floor
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Movement

Irregular echinoids:

  • infaunal mode of life
  • live in burrows or on the sandy sea floor and their tests may be heart shaped rather than circular, showing bilateral symmetry which is better suited to moving and feeding in the sediment
  • some flattening of the test may also be evident as an adaptation to life in the sediment
  • the spines on these echinoids tend to be shorter and more closely set together like a coarse fur and are used either for digging or for moving currents around in the burrow
  • irregular echinoids can move forward through soft sediment by the rowing action of the flattened spines of the plastron
  • the ambulacral plates and hence the tube feet of the irregular echinoids do not extend to the underside of the test but form a petaloid pattern on top of the shell instead
  • this distribution reflects their function as food gathering and respiration organs in the irregular echinoid
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Feeding

Regular echinoids:

  • have their mouths in the centre of the underside (oral side) of the test
  • they have a large peristome to support a heavy jaw sysetem which they use to graze on seaweed as well as for scavenging on dead organic remains

Irregular echinoids:

  • filter feeders and have lost the need for a large jaw system to devoiur food
  • as a result they have a smaller peristome reflecting the lighter system of jaws and the mouth is rarely central but has moved forward to the front (anterior) of the underside (aboral side) of the test
  • in some forms there is a lip or labrum which helps to direct food to the mouth 
  • the spines of the fasciole are used to create downward currents in the burrow which pass water over the tube feet of the ambulacra which then pass food to the mouth
  • the anterior groove also aides the flow of currents to the mouth
  • currents also sweep over the petals where the tube feet have a repiratory function 
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