Sea Lilies and Feather Stars: Crinoidea

General body plan and external features

Of about 630 extant species of crinoid, about 80 are stalked crinoids or sea lilies, the remainder are non-
stalked
feather stars (comatulids). There are more than 5000 species of extinct crinoid. Crinoids have a
jointed or scaly appearance. Sea lilies are divided into the stem (stalk or column), which has a cylindrical or
pentagonal contour and a jointed appearance, and a crown or corona, which bears the arms.

The stem may be up to 50 cm long (up to 21 m in extinct forms) and has an attachment disk or digitated
root-like attachment organ. In some extinct forms the distal end of the stem sometimes possessed a
grappling hook or an end-bulb for attachment, or the stem was slender and prehensile and in some forms
possibly ended in a float.

The stem may possess
cirri which are jointed appendages. At the base of the stem these may also provide
additional anchorage. The cirri are arranged in regularly spaced whorls (2, 3 or 5 cirri per whorl, usually 5).
Comatulids lose their stem during embryonic development and usually have cirri (0-80 cirri). The cirri may
be equipped with aboral spines and may have terminal claws.

The
crown consists of a central, rounded, oval, hemispherical or discoidal mass (containing the viscera)
and attached arms (brachia) arranged pentamerously. The aboral surface forms a cup or saucer (
calyx or
dorsal cup) roofed by an oral membrane (
tegmen, disk, or vault). The mouth is at or near to the tegmen
centre. Five
ambulacral grooves extend from the mouth to the arm bases. The anus is excentric at the tip of
the anal cone (anal tube). When present, the stem is attached to the aboral base of the calyx, so that the
mouth faces upwards.

The plane bisecting the mouth and anus (or alternatively mouth and hydropore) forms ray A, the anterior
ray or radius. Proceeding clockwise, as seen with the oral surface facing the observer, the other four radii
are labelled B, C, D and E. The anus occupies interradius CD. When the mouth is displaced peripherally,
only the ambulacral grooves on the same side of the mouth may remain.

The tegmen is perforated by 500-1500 tiny pores, which form entrances to water canals (ciliated funnels)
leading into the coelom. The brachia sprout from the calyx-tegmen boundary. These arms are also jointed
or scaly in appearance. There are five arms, but these may fork into ten and may branch 8-9 times to give
40-200 branches. The arms may be long and slender or short and broad. Warm temperatures favour long
branching arms, whilst cold temperatures favour fewer shorter arms. The arms may vary from 10 mm to 300
mm in length.

A row of short, jointed side branches, known as
pinnules, occur on each side of each arm. The proximal or
oral pinnules are tactile and protective, they have no ambulacral grooves, no podia and are long, rigid and
spine-like and exhibit limited movements. They may be equipped with a terminal comb of teeth of unknown
function.  The middle pinnules are the genital pinnules and contain the gonads, which become swollen at
maturity (at other times these pinnules resemble the distal pinnules). These pinnules are short and slender
and may possess podia and grooves. The distal pinnules are long, slender and possess podia and
grooves. These grooves branch from the main arm grooves. The most distal 2-4 pinnule joints always bear
aboral hooks. Pinnules may or may not possess spines.

The colour of crinoids tends to decrease with depth. Comatulids are especially colourful and may be: white,
cream, yellow, orange, green, olive, bright red, wine red, maroon, purplish red, purple, violet, brown or
black. Some have two or more colours and some species have numerous colour variants.

Body wall

Epidermis may be incomplete or absent over most of the body surface, and may be syncytial. The
epidermis lacks a basement membrane and is ciliated only on water canals and ambulacral grooves.

Endoskeleton

Ossicles occur in the dermis, as in all echinoderms. The ossicles have a fenestrated mesh structure and
are: 83-91% calcium carbonate, 7-13% magnesium carbonate, 0.02-5.7% silicon dioxide, and < 1% metallic
oxides and contain traces of calcium phosphate.

The stalk ossicles form a single row of rounded or pentagonal diskoid or cylindrical columnals (5 rows
ancestrally, became fused??). If present, then whorls of cirri attach to columnals called nodes, which
possess articulatory facets for the columnals. The other columnals form the internodes (1-45 columnals per
internode). The stem grows by adding new columnals to the top, just below the calyx, and by interpolation of
columnals into internodes. The maximum number of internodal columnals is typical of the species. The
columnals articulate by lock-and-key style articulations, which may be pentametrously arranged.

The stem lacks muscles, but has elastic fibres connecting adjacent columnals. However, the stem is
capable of some movement, enabling it to position the crown against the current (see ‘nutrition’ below).
Rigid joints, called syzygies, connect nodes to internodes. These are connected by very short elastic fibres.
The stem is most prone to break at these junctions. A central canal runs through the columnals (a small
central hole) and carries coelomic canals and nerves.

The cirri contain skeletal ossicles called cirrals (15-50 cirrals per cirrus). These also contain central canals
continuous with those of the column.

The calyx contains 2-3 alternating pentamerous cycles of skeletal plates. Monocyclic forms have two cycles
(with stem-angles at radii): the aboral basal plates and oral radial plates. Dicyclic forms have three cycles
(stem-angles interradial): additional infrabasal plates are aboral to basal series. Additional perisomatic
plates may occur between the radials and arm bases (interradials and interambulacrals) and connected
with the anal cone (e.g. radianal plates). Plates within a cycle may fuse to give 1-4 plates. Extant species
are all either monocyclic or pseudomonocyclic. The latter appear cyclic because the infrabasals are much
reduced.

In comatulids, the basals are reduced or internalised in comatulids, forming a decagonal disk. The
centrodorsal ossicle is the top columnal and is retained to form a major part of the calyx and bears the cirri
(if present). The radials form a radial pentagon. Comatulids possess a skeletal structure called the rosette,
formed by fusion of the greatly reduced basals. The rosette has a hole in its centre and forms an oral roof
to the centrodorsal. The oral face of the centrodorsal usually has 5 radiating interradial ridges or grooves
between which the radials fit. Sometimes five rods, the basal rays, extend from the rosette along the radial
grooves or ridges of the centrodorsal.

Brachials are the ossicles in the arms. Primibrachs are ossicles in the five main arm trunks, secundibranchs
occur in the ten forks and tertibrachs or palmars form the third branches and postpalmers form the other
smaller branches. The brachials may be uniserial or may zig-zig in alternate fashion to give a biserial or
intermediate uniserial/biserial appearance.

Brachial ossicles may form movable articulations with flexor muscles opposing elastic ligaments, or
immovable joints. Of the immovable joints synarthrie are flecible, bound together by elastic ligements, and
syxygies are rigid and connected by short elastic filaments. These can be seen as wavy lines joining a distal
epizygal ossicle to a proximal hypozygal ossicle.

The tegmen may or may not contain endoskeletal ossicles. The ancestral type of five deltoid plates may be
retained, or there may be many small tegmen plates or microscopic calcareous inclusions.

The pinnules contain ossicle called pinnulars. These are moved by muscle/ligament antagonistic
articulations as in the brachials. Muscle contraction causes flexion, which stretches the elastic ligaments,
which hence contract when the muscles relax. The cirri are held together by elastic ligaments only, not
muscles, but can still move, suggesting that these ligaments have some innate contractile powers.

The ambulacral grooves reside in deep depressions on the oral surface of the brachials and pinnulars. The
edges of the grooves may be raised to form repeating lappets, which have a scalloped contour, alternating
on both sides of the groove. Lappets may close over the groove and podia for protection. Calcareous
bodies (spicules, rods, fenestrated plates, etc.) may occur in the soft tissues.
crinoid
Crinoids - Sea Lilies
Tube feet

A ring of 20-25 oral or labial podia encircles the mouth (borne on the outer rim of a pentagonal ambulacral
depression). Tube feet also occur in the ambulacral grooves of the arms and are considered under
‘nutrition’. Appendages called saccules: small spherical bodies containing protein, also border the
ambulacral grooves.

Coelom

The coelom is lined by a cuboidal epithelium. Rather than a single large cavity the coelom is divided by
strands, webs and membranes of connective tissue that fill much of the body. (These may contain calcium
carbonate inclusions). The coelomic axial sinus surrounds the oesophagus and is enclosed by intestinal
coils. This sinus, and other coelomic spaces, continues into the arms and pinnules. (There are five
coelomic canals per arm – one aboral canal, two subtentacular canals, a genital canal and a tiny fifth canal
between the water canal and ectoneural nerve band). Cilia are lacking except in 2-6 ciliated pits in each
pinnule, sometimes these occur in the arms.

The chambered organ is aboral to the axial sinus (in the cavity between the rosette and inner surface of the
centrodorsal plate in comatulids, and in a similar position in sea lilies). This organ consists of 5 coelomic
cavities pentamerously arranged (interradial in monocyclic forms, radial in dicyclic forms). These chambers
terminate orally, but continue as canals into the cirri at each node. The chambers are ensheathed in
nervous tissue.

Water-vascular system

This system has no direct contact with the outside in crinoids and is coelomic. Canals run along the arms,
under the ambulacral grooves, and along the pinnules to the podia. They also run along five main radial
grooves beneath the ambulacral grooves of the tegmen. A ring canal (pentagonal) encircles the mouth
beneath a pentagonal ambulacral depression. The ring canal gives off canals to the labial podia. About 30
stone canals per interradius open to the coelom.

Haemal system

The perioesophageal haemal plexus innervates the spongy organ, which may be the site of coelomocyte
production. A subtegminal plexus supplies the genital tubes.

Axial gland

The axial gland resides inside the coelomic axial sinus and accompanies the spongy organ. Aborally the
axial gland passes through the central hole of rosette in comatulids, and enters the central canal or cord of
the chambered organ. Orally it terminates near the mouth, close to haemal plexi. The lack of a discharge
duct for the axial gland suggests that it has an endocrine function.

Nutrition

Crinoids are suspension filter feeders. When feeding, the arms and pinnules are held outstretched with the
tentacle-like podia erect. The podia possess mucus-secreting papillae along their length. The podia are
arranged into triplets. The primary podium of each triplet is long and when it contacts a food particle it whips
into the ambulacral grooves and is wiped against the ciliary current or against the short tertiary podia or
between adjacent lappets. The secondary podia function like the primary podia, but are shorter. The food
particles are subsequently transported down the food groove by the ciliary current.

The principal food depends on species, but includes zooplankton and detritus (including bacteria). The
filter-fan is oriented into the current by bending of the stalk. The fan is often at 90o to the current,
depending on lift forces, but some crinoids form a vertical collecting funnel to catch sedimenting particles. In
crinoids concealed in protective crevices the arms may simply extend in several directions. The pinnules
and podia form a tight mesh and an efficient filter. The total ambulacral length may be as much as 80 m!

The mouth opens into a short oesophagus that connects to the intestine. The intestine undergoes a
complete turn inside the calyx and may be enlarged and may possess lateral outpouches. In those forms
with an excentric (exocyclic) mouth at the periphery of the disk, the intestine may form four coils, which are
no wider than the oesophagus. The intestine opens through the anus in the anal cone via a short rectum.
The anus discharges large, compact, mucus-cemented balls that sediment from the water column.
Crinoid vertebrae
Nervous System

There are three interconnected nervous subsystems:

1. The oral/superficial/ectoneural system: comprises a neural band beneath the ambulacral grooves of the
arms and cirri, immediately beneath the epidermis, and innervates the podia. The five main bands
converge to the mouth and form a nerve sheath along the wall of the digestive tract.

2. The deeper oral or hyponeural system: comprises a pentagon in the connective tissue of the tegmen,
lateral to the water-vascular ring canal. It gives out nerves to the tegmen podia, the anal cone, the internal
organs and ten radial nerves to the arms. These ten nerves fork to give two nerves per arm. These
innervate the water-vessels, pinnules and podia.

3. The aboral or entoneural system: the main part of the nervous system in crinoids. (The oral system
dominates in other echinoderms). It forms a cup-shaped mass in the apex of the calyx cavity and gives out
nerves to the cirri (in comatulids) or to the column and hence also to the cirri at the nodes (in sea lilies). It
gives out 5 brachial nerves to the arms. These contain ganglia that innervate the flexor muscles.  The
aboral nerve mass also gives out 5 lateral trunks, which fork into ten and are united by a pentagonal
commissure (concentric with the main nerve mass) in the radial plates of the calyx.
Locomotion

Comatulids swim by moving the arms up and down in alternate sets of 5. During the upstroke the pinnules
are folded and during the power stroke the pinnules extend. Maximum speeds are about 5 m / min. and a
maximum distance of 3 m can be covered in a burst, before rest is required. When gaining initial lift the
arms may beat as fast as 100 rpm. Comatulids can also move by creeping, using the arms to pull
themselves along at about 40 m / h.

Reproduction

The gonads are contained in the arms or genital pinnules. Sex cell masses fill the genital cavities. Crinoids
are dioecious, and the sexes are indistinguishable except for the presence of brood chambers in the
females of some species. The genital canal is a coelomic canal in each arm, containing a genital cord or
strand of cells that transport the primordial sex cells to the gonad. Sometimes the genital cord is contained
within a genital tube, which is possibly a haemal sinus. The sex cells are liberated by rupture of the pinnule
wall. Some crinoids brood their eggs, Antedon attaches the eggs to the pinnules, while some Antarctic
comatulids retain the eggs in pinnule brood chambers, in which the larva develops to the stalked stage.
Male spawning has been observed to trigger female spawning. Some crinoids are viviparous, though the
mechanism of sperm entry to achieve internal fertilisation is not understood.

Embryology

Development has been well studied in the comatulid Antedon. The egg undergoes indeterminate,
holoblastic and radial cleavage to form a coeloblastula. After gastrulation the pore closes, cutting off the
archenteron (after about 36 h), which later forms the gut and coelom. The first larval stage is the pelagic
doliolaria, with 4 ciliary rings. After attachment to the substrate, the doliolaria metamorphoses into a stalked
cystidean larva (resembling a cystoid). After a few days the mouth opens and 5 arms are produced,
resulting in a pentacrinoid larva, which gives rise to the adult.

Ecology

Comatulids occur in depths of  1500 m, whilst sea lilies are only found in deep waters at depths of 200 –
5000 m (occasionally up to 50 m). Crinoids support a range of parasites, inc. copepods, isopods, snails
and the unusual myzostome polychaetes. The myzostomes barely resemble worms and curiously they
induce the formation of galls in infected crinoid tissue. The form and position of these galls varies, and one
is reminded of oak tree galls. As already discussed, crinoids are efficient filter feeders, consuming
sedimenting organic matter and detritus bacteria.
Crinoid arm
Stalked crinoid
Crinoid 3
Crinoid 4
Crinoid 5
A 3D computer model of a stalked crinoid, parsed and rendered in Pov-Ray. One version of the
model has unbranched arms, whilst in a modified version the arms fork - bath these patterns and
multiple branching of the arms occur in different species.
In some species of crinoid the stalk may possess whorls of movable tendrils called cirri (singular cirrus) as in the models shown here. Cirri often end in a claw
and are used to grasp nearby objects for support. Cirri regenerate readily if lost. The end of the stalk may be equipped with a
holdfast consisting of root-like
extensions or disc-shaped expansion fastened to the substrate. In other forms it may simply taper to a free end which can be wrapped around other objects for
support. In some extinct forms the stem may have ended in a many-chambered gas-filled float. In extant forms the stem reaches about 50 cm in length, but in
fossil forms may exceed 20 metres. The arms (
brachia) are five in number in some species, but more usually they fork to give ten branches and in some
species the arms may branch repeatedly, up to nine times, giving as many as 200 branches. In 'stalkless' forms the stalk is typically reduced to a single
disc-plate bearing cirri.
Uiniticrinus is an extinct form that was stalkless and floated in swarms with the arms directed downwards and spreading to 2.5 m. The
crown (central body) consists of rings of armoured plates forming the lower cup (
calyx) and a dome-like tegmen of smaller plates bearing the mouth.
Crinoids are also called sea-lilies because of their flower-like appearance when the brachia are open. The brachia
have rows of projecting, jointed appendages called
pinnae. The pinnae can close together and the brachia can close
together, for protection, in which state the crinoid resembles a closed flower bud. The arms and pinnae are used in
filter-feeding, sieving food from the water. Deep sea forms collect food particles that fall from the waters above, like
snow. Other crinoids are rheophilic (flow-liking) and angle their filter-fan of out-sreteched arms and pinnae into the
water current to strain out any food swept along in the currents.