What is an Echinoid?

What is an Echinoid?

Despite their alien appearance, echinoids, or sea-urchins as they are better known, are very common in the seas and oceans of today and are common fossils too. Their name derives from the Greek 'echin' ('spiny'), referring to their protective spines and presumably 'ooid' (egg-like) in reference to their globular shell, or test as it is known. Echinoids are part of a much larger group of animals known as the Echinoderms ('spiny-skins'), which also includes the Asteroids (starfish), Holothurians (sea cucumbers), Crinoids (sea lilies and feather stars) and the Ophiuroids (brittle stars).

Though their body plans are varied, all echinoderms possess key features which unite the group;

• A complex skeleton of calcareous plates, with a unique spongy structure known as stereom
  
• Five planes of symmetry, referred to as penta-radial
  
• An internal hydrostatic (water-vascular) system, external extensions of which are used for locomotion, respiration and feeding
  
• All live in marine waters
  

Echinoderms first appeared in the fossil record in the Cambrian around 530 million years ago and quickly diversified into many groups. Echinoids appeared in the Ordovician (around 450 million years ago (mya) but were not very successful at first and other groups such as crinoids dominated the Palaeozoic. By the beginning of Mesozoic (250 mya) many of the earlier echinoderm groups were extinct or in decline and the Echinoids rose to abundance. They diversified through the Jurassic (210-145 mya) and have remained successful ever since.

Why are echinoids important?

Echinoids are very useful to palaeontologists because of their functional morphology; basically this means that by studying their anatomy you can tell a great deal about their mode of life and the environment in which they lived. They are also very common, and their robust tests and spines are easily fossilised and collected. Anyone who has hunted for fossils in the Jurassic and Cretaceous rocks of the UK will no doubt be very familiar with echinoids.

Regular or Irregular?

Echinoids fall into two categories; regular and irregular. This isn't referring to how common they are, rather to what shape they are. Regular echinoids have no front or back end and can move in any direction. Irregular echinoids have a definite front and back and do move in a particular direction. This is because regulars and irregulars have very different ways of life. Irregulars evolved from regulars, and their anatomy is therefore a modified version of the regular anatomy. For this reason we will deal with regulars first.

Regular Echinoids

As mentioned above, regular echinoids have no front or back end. Instead, the opening for the mouth (peristome) is on their underside, and the opening for the anus (periproct) is on top. When viewed from above or below their profile is circular and radially symmetrical; hence the term regular ('repeating', 'uniform'). This is because regular echinoids roam the surface of the sea floor in search of food and need to be able to move in any direction. This mode of life leaves them very exposed to predators and they have evolved elaborate spines both for defence and to act as stilts for locomotion.


Most echinoids quickly loose their spines after death, and most fossil echinoids either comprise of isolated tests or solitary spines. However, if you are very lucky you may find a fossil echinoid which has been buried alive and still has its spines attached.


Spines vary greatly between species; some are needle-like, others club-like, some are poisonous and others possess thorny barbs. It is often easier to identify a species by its spines as opposed to its test, as they are much more variable and distinctive. Stripped of spines, many regular echinoids are superficially very similar.

Once a regular echinoid finds food it can grasp it with its tube feet. These are tiny, fleshy suckers which extend from the hydrostatic water-vascular system through holes in the test. The holes through which they extend are organised into five distinct bands, called ambulacra, which run vertically between the peristome and the periproct. Regular echinoids are largely scavengers and have a varied diet of plant matter, animal detritus, sponges, molluscs and barnacles.


Regular echinoids possess large, powerful, and highly complex jaws, known as Aristotle's Lanterns, which extend through the mouth to collect food or scrape organics from shells or other hard surfaces. The jaws are complex beaks with five teeth. These leave a distinctive star-shaped grazing trace called Gnathichnus pentax. Isolated echinoid jaw-parts are not uncommon fossils, but because they are unfamiliar to most collectors they are either overlooked of misidentified.


Echinoids are armoured with minute defensive spines called pedicellariae. These are venomous or possess pincers to deter parasites and clear detritus. Pedicellariae are only preserved in exceptional circumstances.



Irregular Echinoids

The first echinoids were regulars, and irregulars did not evolve until the Jurassic. Irregulars are much more common fossils though, and unlike regulars their tests are typically fossilsed complete. Their spines are almost never found attached. Common forms are sea potatoes and sand dollars.


Irregulars lead a very different lifestyle from that of the regulars. They burrow along the sea floor and bulk-feed on the sediment to extract nutrients. For this reason the radial symmetry inherited from the regulars has been modified; the mouth moving to the front of the animal to collect food and the anus moving to the rear to leave waste behind. Hence their form is now irregular, with only one plane of symmetry.

The spines have lost their defensive role and have become reduced and hair-like. They now help to form the burrow, move the echinoid through the sediment, gather food and generate circulatory currents within the burrow. Many irregulars have lost their jaws as they are unnecessary to their mode of life. The tube feet are modified into flanges for respiration and gathering food, and the ambulacra are often sunken to form a petal shape on top of the echinoid.

Class Asteroidea
Characteristics

• Usually sedentary along shorelines
  
• Starfish or sea stars
  
• Come in a variety of colors
  
• Prey on bivalve mollusks such as clams & oysters
  

Starfish Feeding on Clam

• Have 5 arms that can be regenerated
  
• Arms project from the central disk
  
• Mouth on oral surface (underside)
  

Class Holothuroidea
Characteristics

• Includes sea cucumber
  

• Lack arms
  
• Shaped like a pickle or cucumber
  
• Live on ocean bottoms hiding in caves during the day
  
• Have a soft body with a tough, leathery outer skin
  
• Five rows of tube feet run lengthwise on the aboral (top) surface of the body
  
• Have a fringe of tentacles (modified tube feet) surrounding the mouth to sweep in food & water
  
• Tentacles have sticky ends to collect plankton
  
• Show bilateral symmetry
  
• Can eject parts of their internal organs (evisceration) to scare predators; regenerate these structures in days
  

Structure & Function of Starfish
Body Plan

• Range in size from 1 centimeter to 1 meter
  
• Mouth located on oral surface (underside)
  
• Have an endoskeleton made of calcium plates
  
• Sharp, protective spines made of calcium plates called ossicles found under the skin on the aboral (top) surface
  

• Have pedicellariae or tiny, forcep-like structures surrounding their spines to help clean the body surface
  

Water Vascular System

• Network of canals creating hydrostatic pressure to help the starfish move
  

• Water enters through sieve plate or madreporite on aboral surface into a short, straight stone canal
  
• Stone canal connects to a circular canal around the mouth called the ring canal
  
• Five radial canals extend down each arm & are connected to the ring canal
  
• Radial canals carry water to hundreds of paired tube feet
  

• Bulb-like sacs or ampulla on the upper end of each tube foot contract & create suction to help move, attach, or open bivalves
  
• Rows of tube feet on oral surface (underside) are found in ambulcaral grooves under each arm
  

Feeding & Digestion

• Tube feet attach to bivalve mollusk shells & create suction to pull valves apart slightly
  
• Starfish everts (turns inside out) its stomach through its mouth & inserts it into prey
  
• Stomach secretes enzymes to partially digest bivalve then stomach withdrawn & digestion completed inside starfish
  

Other Body Systems

• No circulatory, excretory, or respiratory systems
  
• Coelomic fluid bathes organs & distributes food & oxygen
  
• Gas exchange occurs through skin gills & diffusion into the tube feet
  
• No head or brain
  
• Have a nerve ring surrounding the mouth that branch into nerve cords down each arm
  
• Eyespots on the tips of each arm detect light
  
• Tube feet respond to touch
  

Reproduction

• Separate sexes
  
• Two gonads (ovaries or testes) in each arm produce eggs or sperm
  
• Have external fertilization
  
• Females produce up to 200,000,000 eggs per season
  
• Fertilized eggs hatch into bipinnaria larva which settles to the bottom after 2 years & changes into adult
  
• Asexually reproduce by regenerating arms