ECHINODERMS: Phylum:Echinodermata Class: Echinoidea Crinoidea (sea urchins) (sea lilies) Order:Regular Irregular

PHYLUM: ECHINODERMATA:  Appeared in the Lower Palaeozoic and have forms still living today.  They are marine with a calcareous skeleton.  Ideally 5 fold symmetry. §Examples include sea urchins, starfish, brittle stars, sea cucumbers and crinoids.  The skeleton (TEST) is internal and is constructed of individual calcite plates.  There is a hydraulic system of tube feet.

CLASS: ECHINOIDEA §They are exclusively marine in shallow depths to the abyssal planes. MORPHOLOGY:  Draw the diagram on page 175 (Black) Figs. a and j. §They have a hard shell which when alive is covered by a very thin skin and therefore they have an ENDOSKELETON.

Echinoid Morphology 2 The skeleton (TEST) is made of calcite with tiny interlocking plates which protect and enclose most of the soft parts inside. The test is usually hemispherical, the interlocking plates are arranged in 10 double columns radiating out from the top of the upper surface (CORONA). See page 175 Fig. a. There are two types of plate: AMBULACRUM INTERAMBULACRUM

Echinoids Morphology 3 Ambulacrum: These occur where the TUBE FEET are positioned. These feet are connected to the WATER VASCULAR SYSTEM (system of hydraulic tubes) through which water is circulated around the body and can be used to extend the tentacles through the test and can act like feet.

Echinoid Morphology 4 Towards the top (APEX) of the test is the APICAL SYSTEM which is made up of about 10 small plates that are interconnected.  Page 175 Fig. 106 c. One plate has a special function: it is porous and allows sea water into the body = MADREPORITE.  This water then passes through the RADIAL CANALS and into the tube feet.

ORDERS OF ECHINOIDS:  Echinoidea are divided into 2 orders which can be achieved by looking at their symmetry: REGULAR ECHINOIDS:  They are usually circular when viewed from above.  They show a 5-fold symmetry. Therefore they have a regular pattern.  The apical system is situated on the top and contains the anus in the centre surrounded by the PERIPROCT (membrane).

Regular Echinoids 2  The mouth is situated on the underside (ORAL SURFACE) usually in the centre.  JAWS are present although they are rarely preserved.  The upper surface is called the ABORAL SURFACE.

IRREGULAR ECHINOIDS:  Look at Page 190 (Black) Micraster and copy the diagram.  These are not circular but are either flattened or heart shaped. They still have 5 rows of ambulacrum and interambulacrum plates but instead of 5-fold symmetry they show a bilateral symmetry.

Irregular Echinoids 2 The ANUS is not enclosed within the apical system. Instead it lies either: 1) On the aboral side half way up the side (Posterior). Sometimes in a groove. 2) On the oral surface towards the posterior. The MOUTH is found on the oral surface either: 1) In the centre with jaws. 2) Closer to the front (anterior) without jaws. Therefore it is easier to define anterior and posterior.

Irregular Echinoids 3 Frequently the two rows of pores within the double ambulacrum plate can diverge from each other and then converge lower down the test forming a distinctive pattern called PETALS or PETALOID. Sometimes the posterior interambulacrum area can extend down across the oral surface, this usually occurs when the mouth is posterior closer to the anterior end. This forms a flatish ridge on the oral surface called the PLASTRON. This may project like a lip across part of the mouth: LABRUM.

Echinoids’ Mode Of Life  Varies depending on whether the echinoid is regular or irregular. §All are benthonic, can move and are gregarious.

Regular Echinoids  They are usually mobile, moving about looking for food and protection.  Many are capable of living on hard rocks: anchor themselves to the rocks via tube feet even in relatively shallow water.  Common between the sub littoral zone down to 100 m.  Can also use the tube feet to climb steep rock surfaces.  On sand they use their spines to support them and move themselves using the spines on the oral surface and low down on the aboral.  Could move in any direction.  They eat sea weed but also partly carnivorous: bryozoa and sponges in particular.  Have strong jaws e.g. Echinus lives on rocks.

Irregular Echinoids Mode of life A) FLATTENED TEST:  Draw diagram from page 185 Black  Clypeaster lived partially or completely buried in loose sediment and moved forward by moving spines to plough through soft sediment.  The tube feet extract organic matter from sediment and transfer to food tubes.  Lives in shallow water m.

Irregular Echinoids 2 B) HEART SHAPED:  Micraster and Echinocardium which could be completely buried.  Common down to 50 m but can survive down to 200 m below sea level.  Lived in burrows of soft sediment (Micraster in fine lime mud).  Draw diagram from page 183 Black. § Burrows forwards using spines and tube feet (Mucus can be secreted to help stabilise the sediment to stop collapsing.

Irregular Echinoids 3  Sand etc. is pushed aside and backwards.  Organic matter is extracted from the sediment and the waste disposed behind.  Some food is also obtained from the sea water via a FUNNEL which extends from the burrow.  T he tube feet in the upper areas extend out of the burrow.  Water is drawn into the animal and CILIA help waft it into the tube feet respiratory system. § All are gregarious.

Echinoid history  Upper Ordovician to Recent:  Began in the Upper Ordovician but only a small number.  In the Carboniferous the numbers peaked briefly but reduced during the Permian.  During the Mesozoic (Triassic) the numbers increased again with new species due to a major adaptive radiation after the Permian extinction provided new niches.  They are very rarely found as Palaeozoic fossils as they did not burrow and plates of test not well fused therefore broke up.  Those preserved are usually found in limestone.

Echinoid History 2  Irregular appear in the Upper Jurassic and increase quickly in numbers.  They increase so quickly because they were more efficient food grazers and had improved sanitation with anus removed from the apical system.  Common in limestone particularly chalk.  Still abundant today.  Micraster was a very important fossil as it evolved quite quickly and palaeontologists were able to show it changing its mouth and anus positions over time. §This added proof to Darwin’s theory of evolution.