Sea Urchin, 3D Pov-Ray model
Echinoids - Sea Urchins
Sea Urchin test, 3D Pov-Ray model
There are about 750 species of extant echinoid.

External Characteristics: colour

Urchins are usually plain and dark shades of green, olive, brown, purple, or black. Some are
pale, almost white or red. The spines may be cross-banded or with contrasting tips. Colour may
depend on age.

External Characteristics: Regular Echinoids

Regular urchins are a regular shape: globose, sometimes flattened at poles, some are ovoid.
They move upon their oral surface, which may be more or less flattened or concave. The
aboral surface is arched. The test may be up to 15 cm in diameter (excluding the spines).
Some deep-sea forms are as much as 32 cm across (exc. Spines).

They possess an armature of thickly placed spines. The spines may be short or long (up to 30
cm long). The spines may be all the same size, or the oral and aboral spines may be shorter
than the equatorial / lateral spines. Often small and large spines are intermingled.

The podia are arranged in a pentaradiate manner: there are 5 double rows of podia extending
from the oral region to the apex along the five ambulacra. They may or may not cross the
peristome (the membrane containing the mouth). The five interambulacra are usually wider
than the ambulacra. The podia usually possess terminal discs, though these suckers are
sometimes lacking on the aboral surface. The podia are very flexible and extensible and can
protrude beyond the spines.

The mouth is in the centre of the oral surface, surrounded by a soft membrane – the
peristome. There may be a thickened rim or lip around the mouth. Five pairs of buccal podia
may encircle the mouth. These are short, stout podia and may be chemoreceptive. The
peristome contains embedded plates and it may have small spines and may have pedicellariae.
Around the edge of the peristome, situated in the interambulacra, there may be 5 pairs of small
bushy gills, which may sit in gill slits (gill cuts) and are presumably respiratory.

The anus may is either in the centre of the aboral surface, or excentric and sits on a soft  
centric membrane – the periproct. This may also have small spines and may have pedicellariae.

The shell, or test, is the body wall containing immovably fused calcareous plates, which support
the spines. However, in some echinoids the test may be leathery and flexible. The ambitus is
the equatorial circumference / contour of the test and is usually circular or slightly pentagonal
or oval.
Regular echinoids: Test Morphology

The test is made-up of 20 curved rows of plates. There are 5 ambulacra with 2 plate rows each
and 5 interambulacra, also with 2 plate rows each. Pores for the podia pass through and not
between the ambulacral plates. Spines are mounted on the tubercles of the test. Primary
tubercles form meridional rows and are largest at the ambitus, smaller at the poles. Smaller
tubercles occur between the rows of primaries. The tubercles consist of a basal boss (a low
truncate cone) and a terminal knob (mamelon) which articulates with the spine. The areole is
the bare area encircling the boss and is the site of spine muscle attachment. A ring of
secondary tubercles may surround the areole (the scrobicular tubercules). These bear small
secondary spines (scrobicules).

The plates are largest at the ambitus and smaller at the poles. They are 5-sided and elongated
horizontally. The plates of each double row alternate, forming a zigzag line where they meet.
The outer edges are straight. The plates are held together by ligaments. Aboral plates
surrounding the periproct form the apical system. This consists of 5 larger genital plates
(interambulacral) each pierced by a gonopore and 5 smaller terminal or ocular plates
(ambulacral) each pierced by a small pore for a modified terminal podium. One of the genital
plates is enlarged into a multiporous madreporite.

The perignathic girdle is a clacareous ridge on the inner side of the peristomial perimeter for
the attachment of the masticatory apparatus. In cidaroids the ridge is well developed at each
interambulacrum, forming a pair of apophyses. In all other echinoids the ridge is best
developed at each ambulacrum where a pair of projections called auricles are formed. These
auricles may meet, forming an arch over each ambulacrum.

The test is usually rigid, however, in the Echinothurridae the test is comprised of overlapping
plates and is flexible. It is moved by a special set of body-wall muscles.

In echinoids two canals connect each podium to its ampulla, so there is one pore pair per
podium. Primary plates contain one pore pair per plate (this is considered the original
condition) whilst compound plates, which are composed of several merged primary plates, have
more pores. Compound plates may be oligoporous (2-3 pore pairs per plate) or polyporous (>3
pore pairs per plate).

There are several different arrangements of the ambulacral plates, depending on species.
These are:

  1. The diademoid condition in which there are 3 full-sized primary plates per ambulacrum,
    as in Diadema.
  2. In the arbacioid condition a full-sized median primary plate flanked by 2 short demiplates
    forms each ambulacrum, as in Arbacia.
  3. In the echinoid condition each ambulacrum is formed from 2 primary plates with one short
    demiplate between their outer ends, as in Echinus.
  4. Insertion of more demiplates into these three types gives several polyporous types.

Irregular Echinoids: external features

Irregular echinoids have an oval to cordiform to circular ambitus. The body is flattened orally
and may be arched aborally or flattened, as in sand dollars.

The periproct and anus are excentric and displaced along an interambulacrum. This introduces
an axis of bilateral symmetry with anterior and posterior ends. The anterior is ambulacrum D (in
Carpenter’s system) or ambulacrum III (in Lovén’s system) and the posterior is interambulacrum
AB or V. The mouth and peristome may be displaced anteriorly.

The ambulacra are petal-shaped and hence called petaloids and are equipped with respiratory
podia. The ambulacra continue over the ambitus to the peristome. Irregular urchins lack gills.

Irregular echinoids are divided into two groups: the spatangoids or heart urchins and the
clypeastroids or sand dollars, cake urchins and sea biscuits.

Spatangoids (heart urchins)

Spatangoids may be up to 18 cm long. The heart urchins have an oval or cordiform ambitus
with an arched aboral surface. The three anterior ambulacra are short and form the trivium. The
two posterior ambulacra are longer and form the bivium. The anterior ambulacrum (D) is non-
petaloid. The oral ends of the bivial ambulacra expand into a petal-like shape, the phyllode,
around the peristome. The phyllode podia are modified.

In spatangoids the labrum is a posterior lip bordering the peristome. A few large subanal podia
form a single row in each posterior ambulacrum. The podia between the phyllodes and
petaloids are much reduced. There are no locomotory podia. The plastron or sternum is the
wide part of the posterior interambulacrum on the oral / ventral surface that is enclosed by the
two long and narrow posterior ambulacra. It may have special spines and extends from the
labrum in front to the periproct behind.

The spines are small to moderate and usually curved and held parallel to the body surface
(they are ‘combed back’). Clavules are narrow bands of dense minute spines, heavily ciliated
basally and shaped like tennis rackets. They maintain water currents that remove sand grains
from the test. The clavules are grouped into tracts called fascioles. The peripetalous fasciole
encircles the petaloids and crosses the anterior ambulacrum. The internal fasciole encloses the
aboral apex and much of the anterior ambulacrum. The subanal fasciole encloses the posterior
part of the plastron anterior to the periproct and encircles the subanal podia. The anal fascioles
run from the angles of the subanal fasciole along either side of the periproct. The lateral or
lateroanal fascioles extend from the posterior angles of the peripetalous fasciole backward
toward the posterior. Not all fascioles are present in any one species; many have 2-3 with the
subanal and peripetalous fascioles being the most common.

The Pourtalesiidae family of spatangoids are very different in appearance. Some are triangular,
pyramidal or bottle-shaped. The periproct is on the aboral surface of the narrowed end. The
peristome is on the oral surface of the broad end. They lack petaloids and phyllodes. The
subanal fasciole is present. They inhabit deep waters and have fragile, transparent tests.

Clypeastroids (sand dollars, cake urchins, and sea biscuits)

Some sand dollars are as little as 10 mm in diameter. Clypeastroids usually have an oval or
circular ambitus and are greatly flattened orally-aborally, though some are arched dorsally.
They are covered in a fur of short spines. The central aboral apex is surrounded by 5 petaloids.
The peristome is in the centre of the oral surface and the periproct is usually oral. There are no
phyllodes and no fascioles. Some species possess two or more round to elongated holes called
lunules, for example the keyhole urchins.

Irregular urchins: Test morphology

The test of irregular echinoids lacks conspicuous tubercles and is covered with innumerable
small or minute tubercles.

Spatangoid test: Larger tubercles occur between the petaloids in some spatangoids. The
ambulacral plates are all simple primary plates, each bearing one pore-pair in the petaloids,
and a single pore elsewhere. The reduced podia and penicillate podia are uniporous. The
ambulacra are often very narrow in spatangoids. The labrum is a plate forming the posterior
border of the peristome. In some irregular urchins, single interambulacral plates meeting the
peristome between the phyllodes form swollen prominences (bourrelets) which form a flower-like
figure called a floscelle. The test of spatangoids is often thin.

Clypeastroid test: In clypeastroids the test may be very thick and equipped with internal beams
and columns for support. Pore pairs are limited to the petaloids. Numerous small suckered
podia emerge through single pores, on both ambulacra and interambulacra of both surfaces.
Primary plates and demiplates alternate in the petaloid ambulacra.
Nutrition

The mouth opens into the buccal cavity, which leads into the pharynx or oesophagus. The
oesophagus passes through the centre of Aristotle’s lantern and leads to the intestine. Aristotle’
s lantern is the masticatory apparatus and is found in regular and some irregular echinoids. The
intestine turns anticlockwise (viewed aborally) and then turns clockwise. The anticlockwise
portion is termed the small intestine or stomach. The clockwise portion is the large intestine and
leads into the rectum, which opens to the outside via the anus. A blind pouch or caecum is
opens into the gut at the oesophagus / intestine junction.

Food, e.g. seaweed, is held by the podia and spines and gnawed by the teeth. Food contacting
the aboral surface is moved to the mouth by the spines and podia. Pedicellariae may immobilise
and help to hold food. Echinoids may be carnivores, eating weak, sluggish or sessile animals, or
they may be herbivores, but most will eat almost anything. Many are scavengers and some
ingest bottom ooze. Shells are also chewed-up and ingested.

Irregular urchins live in sandy bottoms in burrows lined by mucus secreted by the spines.
Curved lateral spines are used for burrowing. The plastron spines ventilate the burrow. The
pencillate podia of the phyllodes are thrust out through the surface hole and probe the surface,
collecting particles by way of an adhesive secretion. The podia then retract, delivering food to
the spines of the upper lip and labrum and hence to the mouth. Food consists mostly of
diatoms, foraminifera and tissue fragments.

In keyhole scutellids, mucous strands with trapped particles are moved toward the mouth by
ciliary action.
Lantern of Aristotle

This is a complex of muscles and calcareous pieces that chew food. It is pentamerous and
conical. It has an apex of 5 teeth, usually seen protruding from the mouth. There are five main
interradial pyramids, each consisting of two half-pyramids joined by a suture. The short
spaces between pyramids are filled with interpyramidal or comminator muscles (transverse
fibres) by which the pyramids can be rocked upon each other. The aboral end of each
pyramid forms a bar or epiphysis – two per pyramid, which may be sutured together. The
aboral end of the lantern forms the base of the cone. In line with the comminator muscles
there are 5 slender radial compasses and 5 stouter rotules. Each compass is formed from two
pieces – inner and outer halves. The outer half is often forked at the end. The pyramids
support the teeth, which are long calcareous bands in the interior spaces of the pyramids. The
hard oral ends of the teeth project into the buccal cavity. The softer, often curled, aboral ends
of the teeth are enclosed in the dental sac. This sac is a coelomic cavity (formed by
evagination from the pharyngeal cavity) from which the teeth grow continuously.

In total the lantern consists of 40 skeletal pieces: 5 teeth, 10 half-pyramids, 10 epiphyses, 5
rotules and 10 compass pieces. The protractor muscles are a pair of flat bands that extend
from the epiphyses to the perignathic girdle at the interambulacra. These push the lantern
outward, exposing the teeth. Retractors pull the lantern back and open the teeth. These
originate on the auricles of the ambulacra and insert on the lower ends of the pyramids.

Small external and internal rotular muscles connect the epiphyses with the rotules. These
transmit movements of the epiphyses to the teeth. The compasses and their associated
muscles are part of the respiratory apparatus.

The whole lantern and its muscles are enclosed in a coelomic membrane, forming a coelomic
cavity around the lantern. This cavity is continuous with the gill lumina. The compass elevator
muscle raises the compasses, drawing fluid out of the gills. Two depressor muscles per
compass depress the compass, forcing fluid into the gills. Thus, the gills are mechanically
ventilated by the piston action of the compasses. The elevator muscle is a flat pentagonal
muscle encircling the oesophagus and attached to the compasses. The depressor muscles
attach the outer end of the compass to the outer surface of the lantern protractors and
originate on the perignathic girdle of the interambulacra.
Body wall

The glandular epidermis is single-layered, cuboidal to columnar and ciliated, except on the
podial suckers / discs and other exposed places (which may be covered by a cuticle). The
epidermis also covers the spines. The epidermis is underlain by the dermis, containing the
embedded ossicles or skeletal plates. This is underlain by a flagellated coelomic lining.
Pedicellariae

There are several types of pedicellariae found in echinoids:

  1. Tridentate or tridactyle pedicellariae, are the largest and are very common. They have
    a head of 3 elongated jaws or blades, often with serrated edges. Usually the jaws only
    meet distally. Rostrate tridentate pedicellariae have shorter curved jaws, and occur in
    spatangoids.
  2. Triphyllous / trifoliate pedicellariae. These are small and have short broad jaws that do
    not meet distally. Bidentate / biphyllous pedicellariae have 2 jaws and are common in
    sand dollars (clypeastroids). Quadridentates are 4-jawed and occur in the family
    Saleniidae, Quinquedentates are 5-jawed and occur in the clypeastroid family
    Laganidae.
  3. Ophiocephalous pedicellariae. These are found mostly on the peristome. They have
    short inwardly concave jaws with blunt tips. They have a basal arc or handle that
    interlocks, holding the grip when the jaws are closed.
  4. Globiferous pedicellariae are equipped with poison glands. The jaws are armed with
    one or more teeth. They are absent in spatangoids. The claviform type contains stalk
    glands, the head having atrophied, and consist of a stalk only with 3 poison sacs. The
    dactylous variety occurs in the echinothuriidae. These have 4-5 long, narrow jaws with
    terminal discs and poison glands.

Cidaroids lack triphyllous and ophiocephalous pedicellariae and so have two types only.
Globiferous pedicellariae act in defence, and release toxin in response to a chemical stimulus.
The heads may detach and embed in attacking starfish. The tridentate and ophiocephalous
pedicellariae are less toxic. The body fluids and axial glands of echinoids may also be toxic.

Sphaeridia

These are minute glassy, transparent, hard, solid, oval or spherical bodies on the ambulacral
areas (except in cidaroids). They are usually stalked and there is one to many per
ambulacrum. In irregular echinoids they sit in cavities, depressions or grooves. They are
thought to be organs of equilibrium.

Gills

All regular urchins except the cidaroids possess gills. Gills are absent in irregular echinoids.
There is one pair at the oral beginning of each interambulacrum. Their lumens open into the
peripharyngeal cavity.

Podia

Regular urchins have 5 double rows of podia on each ambulacrum from the peristome to the
periproct. They may continue over the peristome to the mouth edge (when there are no buccal
podia). Specialised buccal podia may occur around the mouth. Most of the podia are
locomotory, each with a terminal sucker supported by a ring of internal calcareous pieces.
Calcareous spicules support the stalk.

The aboral podia may lack suckers and are sensory papillate podia. Cidaroid podia have
terminal suckers, but locomotion is due primarily to the spines. In spatangoids locomotion is
also due to the spines.

Spatangoid podia

The aboral petaloids have large, thin-walled, leaf-like, lobulated branchial podia, which lack
skeletal support. These may function as gills. The frontal podia of the non-petaloid anterior
ambulacrum are tapering or topped with a scalloped or stellate disc.

The phyllode podia are penicillate (resemble a fruiting Penicillium mould) with an expanded
end covered with erect club-shaped projections, each supported by an internal skeletal rod.
They are chemoreceptive and assist in food capture.

From the phyllodes along the ambulacra in the aboral direction up to the petaloids, the podia
decline to very small, slender forms, except for a few large subanal podia enclosed by the
subanal fasciole. These resemble penicillate podia or frontal podia.

Clypeastroid podia

There are two main types of podia in the clypeastroids: large simple or lobulated brachial
podia on the petaloids and very numerous, small, suckered podia, which cover much of the
test, both ambulacra and interambulacra. These small suckered podia assist the spines in
locomotion and gather food.

Musculature

Due to the rigid body wall, body musculature is absent, except in the echinothuriidae, which
have soft deformable tests. Muscles move the moveable appendages (spines, pedicellarise)
and the lantern.

Echinoids: nervous system

The nervous system is similar to that in holothuroids. The main ectoneural system consists of
the circumoral nerve ring, radial nerves and the subepidermal plexus. The radial nerves
ascend along the midline of the ambulacra along the inner surface of the body wall. The
deeper oral or hyponeural system is present in those echinoids with an Aristotle’s lantern. Five
plaques of radial nervous tissue on the aboral surface of the nerve ring send nerves to the
lantern (and its muscles?).
Sense organs

Spines, podia and spines are all sensory. There are also dispersed epidermal sensory cells.
Sphaeridia may function as organs of balance. In the diadematidae, bright blue spots on the
genital plates, in rows along the interambulacra, often on the peristome and sometimes on the
ambulacra. Sometimes these spots are fused into stripes. These are thought to be compound
eyes. In Astropyga radiata stalked oral blue spots are thought to be photoreceptors.

The subepidermal plexus is photosensitive and echinoids are negatively phototactic. The
aboral surface may be covered with pieces of plants, shells, small stones, etc. held by the
podia, as a protection from light. Shadows passing over urchins ellicit a defencive spine
erection reflex, which involves radial nerve activity. The spines may converge and point
towards the object casting the shadow. Eyes are not necessary for this reflex; the test surface
(not the spines) contains the photoreceptors. Keyhole scutellids undergo diurnal colour
changes, lightening in the dark.

Coelom

There is a spacious major cavity. In addition there are several minor cavities: the
peripharyngeal cavity, periproctal sinus, perianal sinus and the genital sinus. The
peripharyngeal cavity encloses the lantern and may give off 5 radial sacs (Stewart’s organs)
which may function as expansion chambers.

The coelomic fluid contains about 4 x 103 coelomocytes per mm3. These are amoebocytes
and a small number of flagellated cells. The flagellated cells may be peritoneal cells detached
from the coelomic lining. They are thought by some to give rise to amoebocytes, though the
dermis is also known to be a site of coelomocyte production. Some of the amoebocytes are
phagocytic with either pointed pseudopods (filopods) or petallate pseudopods. Others may
contain inclusions, including clear granules that give rise to melanin on cell breakdown. Some
amoebocytes contain red echinochrome pigment.

Water-vascular system

This has the usual echinoderm plan. A main water ring around the digestive tube (where it
emerges from the lantern) gives rise to a stone canal, which ascends to the madreporic plate.
An axial gland accompanies the stone canal. Interradial branches connect to five polian
vesicles or spongy bodies. Alternatively there may be a continuous spongy ring in some
clypeastroids. Five radial water canals follow inside the ambulacra, giving off branches to the
ampullae. Each radial canal ends in a terminal tentacle.

Axial gland

This accompanies the stone canal and is comprised of spongy tissue. There is no axial sinus in
echinoids. The axial gland is hollow, due to an internal coelomic cavity, and is well supplied by
the haemal system. The axial gland is thought to be a point of communication between the
haemal and water-vascular systems. It consists of a meshwork containing coelomocytes.

Haemal system

A haemal ring encircles the oesophagus on the aboral surface of the lantern. This gives rise to
interradial spongy bodies and radial haemal sinuses. Each haemal sinus passes down the
outer surface of the pharynx, inside the lantern, to the peristome and then along the test
radius, giving off branches to the podia. There is an inner or ventral marginal sinus supplying
the large and small intestines, and a smaller outer or dorsal marginal sinus supplying the small
intestine. Presumably these marginal sinuses take-up the products of digestion from the
intestine.

Excretion

Waste-laden coelomocytes accumulate in the gills and body wall or deposit granules in the
body wall. The axial gland is also involved; waste-laden coelomocytes accumulate here and exit
via the stone canal and madreporite.

Locomotion

Locomotion may be affected by podia, spines or both. Spine powered locomotion is faster,
reaching 25-35 mm / s compared to a maximum of 150 mm / min. for podial locomotion. The
lantern may also be used out of water: the urchin lurches forward by pushing with its teeth and
spines. Righting movements occur in 1.5 to 2 minutes in regular urchins, but may take an hour
in sand dollars. Righting in sand dollars is slower without the sphaeridia, but these are not
essential. There may be a slightly preferred leading ray in some regular species. Irregular
urchins only move with their anterior end forward.

Sand dollars move with Loven’s ray III forward, turning rather than reversing. They are
propelled at up to 18 mm / min. on top of the substrate, primarily by spine movement. They
burrow by first building a mound of earth into which they thrust themselves. This whole process
takes 15-20 minutes.

Keyhole scutellids burrow in about 15 minutes by rotating their test from side-to-side, during
which sand is driven through the anterior lunules by spine action. Some species can not right,
but the inverted position is unstable and easily righted by wave action.

Irregular urchins live in mucus-lined burrows. The curved lateral spines are used for burrowing.
Spines secrete the mucus, while plastron spines ventilate the burrow.

Reproduction

Regular echinoids have 5 interambulacral gonads are fastened by mesenteries. Each leads to
an external gonopore, via a gonoduct, on the apical genital plate. Most irregular echinoids
have 4 gonads (gonad along interambulacrum AB is missing), but some have 3 (CD and AB
missing) and others have 2 (CD, AB, DE missing).

Sea urchins are strictly dioecious, but rare anomolous hermaphroditic individuals do occur. A
few species exhibit sexual dimorphism. In Psammechinus milers the male gonopores occur on
short papillae. In Echinocyamus pusillus the male genital papillae are longer. In brooding
spatangoid species the female has deeper petaloids than the male.

Fertilisation is external. Echinoids may aggregate to spawn, which may take place at full moon.
Some pair off for spawning. Spawning occurs around spring and summer in the Northern
Hemisphere.

The gonads have an outer coelomic epithelium, with muscle fibres and connective tissue
underneath and an inner germinal epithelium. Contraction of the muscle fibres causes
spawning. This may be in response to a chemical stimulus since the gonads have no apparent
nerve supply.

Brooding is most common in the cidaroids and spatangoids, especially in the Antarctic. These
species produce large yolky eggs, which develop on the peristome or around the periproct
(sometimes in an annular groove). Brooding spatangoids may have deepened petaloids, which
form a brooding chamber roofed by criss-crossed spines.

Development

The eggs float or sink, depending on species. Cleavage is holoblastic and initially equal, giving
rise to an 8-celled stage with 4 vegetal cells, which give rise to small micromeres and large
macromeres by unequal cleavage, and an animal pole of mesomeres. An 800-1000 cell
coeloblastula is formed, with small cells at the vegetal pole. After about 12 hours post-
fertilisation, a free-swimming flagellated blastula is produced, which develops into a pluteus
larva (an echinopluteus).

The echinopluteus has 4 arms, one pair of postoral arms and one pair of shorter anterolateral
arms. These increase the area of ciliary loops used in feeding on micro-plankton. As it
develops it may develop extra arms, up to a total of 4,5 or 6 pairs. These are the posterodorsal
arms and preoral arms and optionally the anterodorsal and posterolateral arms. The
posterolateral arms may only be present as short processes. The resultant larvae can have
very variable shape and structure. Calcareous rods support the arms.

Diadema larvae have 4 arms, and the horizontal postoral arms greatly elongate into the so-
called plutei transversi. Spatangoid plutei develop an aboral spike (median posterior
projection).

After 4-6 weeks metamorphosis produces a young urchin (takes about one hour) less than 1
mm in diameter. Spines, podia, test, etc. continue to develop. Urchins may live for 8 years or
more.

Ecology

Echinoids are all marine and benthonic. They inhabit all seas and all types of bottom, from the
intertidal zone to about 5000 m. They are apparently absent from the deepest abysses. They
are most common in the littoral zone. Regular urchins tend to prefer hard botttoms, irregular
urchins soft sandy bottoms. Deeper water forms live on the bottom ooze.

Rock-boring urchins excavate burrows in rocks for protection against waves. The urchin may
become permanently trapped in the burrow as it grows. Boring is accomplished by the rotary
motion of spines, assisted by the teeth. These urchins cause damage to steel pilings.

Colobocentrotus (an echinometrid) has large strong oral podia and an ambitus fringed by
broad flat spines, which act as a sucker for firm attachment. The aboral spines serve to break
the force of the waves.

Crabs, sea stars, large fish, mammals and birds may eat urchins. Birds may fly up and drop
them onto rocks to crack them open. Crustaceans and small fish may seek shelter among the
spines of poisonous echinoids. The spines of cidaroids are devoid of a living surface and are
usually overgrown by algae, sponges, hydroids, zoanthids, anemones, bryozoans,
brachiopods, tubiculous polychaetes, and barnacles, etc. Commensal ciliates commonly occur
in the digestive tract. Other protozoa may live in the coelomic fluid and flatworms may live in the
intestine or gonads. Nematodes up to 150 cm long have been found in the coelom. A peculiar
sea cucumber, Taeniogyrus cidaridis, lives on cidaroids, with the posterior part of its body
coiled around the urchin’s spines. Ophiuroids have been found attached near the mouth.
Some bivalves and snails may also parasitise echinoids. These snails may bore into the base
of a cidaroid spine, producing a gall, which contains one or more snails. Crustaceans may also
parasitise echinoids and may induce galls.
Article constructed: 13/6/2017
Above: 1 3D model of a sea Urchin
('reqular' type) seen in side-view. Note
the radial rows of
spines of varying
sizes and of sucker-like
tube-feet.
The tube feet (
podia, sing. podium)of
sea urchins can be extended and are
highly mobile, the rigid spines can be
moved at their bases.

Left: stripping away the 'living parts'
from our model (tube feet, epidermis
and spines) we are left with the sea
urchin test. This model was
constructed plate by plate Note that the
plates arearranged in vertical / radial
rows and each plate bears one or more
bosses, each boss bearing a spine in
life (above model).
Seen from above (the aboral pole) the radial rows of plates become clearer. The widest point
of the urchin is the
ambitus (which would be the equator if they were perfectly spherical). The
five narrower
ambulacra are each made up of two rows of interlocking and alternating
ambulacral plates, which contain minute pores for the tube feet. Alternating with the
ambulacra are the wider
interambulacra, again formed of two rows of interlocking
interambulacral plates bearing bosses but no pores for tube feet. The aboral pole consists
of the central
periproct, a membrane reinforced by calcareous plates, which bears the anus,
either centrally as in this case, or off to one side. Around the periproct is a series of plates
forming the
apical system: larger genital plates, each bearing a gonopore and five smaller
terminal plates each pierced by a small pore for the emergence of a terminal podium which
lacks a sucker and instead taper to a blunt rounded tip and serve sensory functions
(chemosensory?). The genital plates are in line with the interambulacra, the terminal plates
with the ambulacra. One of the genital plates (generally larger than the other four causing a
departure from pentamerous  symmetry in that the other apical plates may not align exactly
with the ambulacra and interambulacra) bears a number of pores is a porous sieve, the
madreporite (madreporic plate), which is the opening to the water-vascular system
through which sea water enters this hydraulic system which operates the tube feet.
Above: The oral pole, underneath the urchin, has a central mouth through which five teeth
protrude. Around the mouth is the
peristome which has embedded plates to support spines
and pedicellariae (see below) and at its margin there is a notch in the ambulacral plate on
each side of each ambulacrum, called the gill cut, through which a gill emerges (so there are
ten gills in total) these provide oxygen primarily for the masticatory system (the lantern of
Aristotle and associated muscles, see below). If you look at dead cleaned-up test, lacking the
peristome, you will see a thickened notched rim bordering the peristome on the inside, this is
the
perignathal ridge to which masticatory muscles attach.
Sea urchin aboral view, 3D Pov-Ray model
Above?: the lantern of Aristotle (Aristotle's lantern) in side-view. This is a complex masticatory
machine inside the test at the oral end, connected to the protruding teeth. This is responsible
for moving the teeth and chewing food as well as circulating coelomic fluid in and out of the
gills  The main body of the lantern consists of 5 pyramids: each pyramid is a pair of plates
fused in the middle (usually) to form a V-shape. Inbetween the neighbouring pyramids is an
interpyramidal muscle, contraction of which rocks the teeth over one-another, creating a
grinding motion to scour the surface the urchin is feeding on..
Above: aboral view of a lantern. The five arched epiphyses are each made-up of a pair of
plates fused (usually) at the midline and these serve as anchors for attachment of muscles.
The
lantern protractors are pairs of flat muscle bands, extending from the ends of each
epiphysis to the interambulacral regions of the perignathic girdle inside the test around the
peristome. Contraction of these muscles lowers the lantern, causing the teeth to protrude
further from the mouth.
Lantern retractors lift the lantern up, raising the teeth, and originate
higher up on the ambulacra, inside the test, and insert at the bases of the pyramids, so that
their contraction lifts the lantern more towards the aboral pole. The five
rotulas, below the
compasses, are connected to the epiphyses by external and internal
rotular muscles, which
transmit forces from movements of the epiphyses to the teeth. The five
compasses, each
consisting of an outer and an inner part fused together, are connected by the
compass
elevator muscles
(which form a pentagonal band of muscle). Contraction of this muscle
raises the compasses, which serves to expand the coelomic cavity which encloses the lantern
and is continuous with the gill cavities, thus raising the compasses draws fluid out of the gills,
ventilating the gills and circulating oxygen in the coelomic fluid to the muscles of the lantern.
Above: a model of a tridentate pedicellaria, closed (left) and open (right).