Echinoderms

Question 1

How are the phylum echinodermata described?

Answer 1

6,000 spp- all are marine, benthic and mostly large (several cm)
Pentamerous radial symmetry
Internal skeleton of calcareous ossicles often project as spines or tubercles
Mutable coonnective tissue- the ossicles are bound together by extracellular collagenous connective tissue that can be rapidly stiffened and made rigid or softened, in some cases to an almost liquid state
Water vascular system and tube feet

Question 2

How is the development of the phylum echinodermata described?

Answer 2

Deuterostomes (radial, indeterminate cleavage and enterocoely)
Larvae are bilaterally symmetrical (many have ciliated bands for locomotion and feeding, some feed from yolk or undergo direct development)
Larva settles and metamorphoses into radially symmetrical adult form

Question 3

What classes exist within the phylum Echioderms?

Answer 3

Asteroidea- starfish, sun stars
Crinoidea- crinoids and featherstars
Ophiuroidea- brittle stars
Echinoidea- sea urchins
Holothuroidea- sea cucumbers

Question 4

How are the class Ateroidea described?

Answer 4

1,500 spp, global distribution, mainly coastal areas
Most species are pentamerous but some have more than 5 arms
The cushion stars have very short arms so look different from more typical starfish

Question 5

How are the class Asteroidea structured?

Answer 5

Mouth is on underside- the underside is termed the oral surface and the upper surface is the aboral surface
Ambulacral grooves along oral surface of arms have 2 or 4 rows of tube feet
Ambulacral spines run along the side of the ambulacral grooves

Question 6

What does the aboral surface of the class Asteroidea contain?

Answer 6

Aboral surface with an inconspicuous anus and a madreporite (part of water vascular system)
Surface may be smooth or there may be various spines, tubercles or ridges
In come species conspicuous marginal plates are arranged along the side of the arms

Question 7

What is the endoskeleton of the echinoderms structured like?

Answer 7

Endoskeleton is a lattice of rods and plates bound by connective tissue- this can be stiffened or softened as required
Each ossicle is formed from a single crystal of magnesium rich calcite

Question 8

What can the surface of the class Asteroidea contain?

Answer 8

Surface structure may include various projecting spines or tubercles
Paxillae- present in some groups of asteroids- raised ossicle with moveable spines on the surface
Adaptation to burrowing existence- respiratory or feeding currents can flow in space below paxillae

Question 9

What is the Pedicellaria?

Answer 9

Present in some groups of asteroids and in some other echinoderms
Small jaw-like appendages on surface- function in protection from small animals or settling larvae

Question 10

How does the water vascular system work in the Asteroidea?

Answer 10

System consists of a network of internal ciliated canals linked to tube feet
Tube feet (podia) are muscular and can bend in different directions- they typically have a sucker at the tip

Question 11

What does the madreporite serve as in the asteroidea?

Answer 11

Madreporite serves as an intake of seawater to replenish leakage at the tube feet
The ampullae are muscular- contract, valve closes to isolate from rest of the system and tube foot extends
Adhesion of tube foot to substrate is mainly chemical- comparable to a duo gland system

Question 12

How do the asteroidea feed?

Answer 12

Mostly scavengers and carnivores- many can evert the stomach to engulf a food item
Some sediment dwellers can burrow down to prey
Some can catch small fish and invertebrates with specialised groups of pedicallaria
Some are suspension feeders- particles trapped by mucus on body surface and transported to mouth by epidermal cilia

Question 13

What do the asteroidea feed on?

Answer 13

Many feed on bivalves- connective tissue stiffens and the starfish skeleton becomes a rigid scaffold enabling them to use the hydraulic force of the tube feet to pull open the valves of bivalve prey enough to insert eversible stomach and begin digestion

Question 14

How are the class Ophiuroidea described?

Answer 14

2,000 spp, most environments from shallow water to deep sea
Similar body shape to asteroids but arms are more sharply separated from central disc and have a more solid structure (appear segmented)
No ambulacral groove and tube feet have limited importance for locomotion
Madreporite on oral surface

Question 15

How do the class Ophiuroidea move?

Answer 15

Highly mobile- push and pull with arms- lateral spines on arms grip substrate

Question 16

How do the class Ophiuroidea feed?

Answer 16

Carnivores, scavengers, deposit feeders or filter feeders- species are generally not restricted to a single feeding mode

Question 17

What are some examples of filter feeding in Ophiuroidea?

Answer 17

Ophiothrix- tube feet form comb-like filters off the sides of the arms
Ophiocomina- mucus suspended between the lateral arm spines trap particles, tube feet collect the mucus as a bolus and transfer via adjacent tube feet towards mouth

Question 18

How may the tube feet be important in feeding in the class Ophiuroidea?

Answer 18

Tube feet may collect food particles directly from the substrate and pass towards mouth
Larger food items may be swept towards mouth by the arms- most carnivorous feeding by this mechanism
Basket stars filter by seizing plankton with tips of their finely divided arms

Question 19

Where do the Ophiuroidea reside?

Answer 19

Many live in crevices of one sort or another
In fast current areas may be epibenthic and very abundant
Some burrow in sediment and only the arms project into the water to feed

Question 20

How are the class Echinoidea described?

Answer 20

950 spp
No arms- body circular or oval
Skeletal ossicles interlinked to form a solid test
Surface covered by movable spines
Regular echinoids (sea urchins) are epibenthic and irregular echinoids (heart urchins and sand dollars) are infaunal

Question 21

What is the structure of the regular echinoids?

Answer 21

Globose test and long spines
Ambulacral areas with tube feet are arranged in meridians around the test and alternate with interambulacral areas
One of the plates near the anus is modifed as the madreporite

Question 22

How do the spines look on the regular echinoids?

Answer 22

Spines articulate on a ball and socket joint
Function in protection and used for wedging into crevices
Movement is facilitated by movement both of the spines and the tube feet

Question 23

How do the regular echinoids feed?

Answer 23

Most are grazers
The mouth has a structure called Aristotle’s latern- 5 radially arranged jaws for scraping food from the substrate
Adapted for burrowing- mostly deposit feeders- tube feet used for food collection and transport
Aboral tube feet function in gas exchange

Question 24

How are the irregular echinoids structured?

Answer 24

Test densely covered in small fine spines- function in burrowing and in protection of body from abrasion from sediment
Heart urchins are more globose in shape and sand dollars are flattened for shallow burrowing

Question 25

How is the symmetry arranged in the irregular echinoids?

Answer 25

Bilateral symmetry due to burrowing habit- anus posterior rather than dorsal, mouth shifted to anterior

Question 26

How are the class Holothuroidea described?

Answer 26

900 spp
Body cylindrical due to elongation along oral/aboral axis
Tube feet around mouth highly modified as buccal tentacles- often branched
Skeleton reduced to microscopic ossicles

Question 27

How is the symmetry arranged in the class Holothuroidea?

Answer 27

Ambulacral areas arranged along meridians as in echinoids
Effectively lie on their side- many have well developed tube feet on the functional ventral surface (sole) and poorly developed tube feet on the dorsal side- bilateral symmetry

Question 28

What are the different lifestyles of the class Holothuroidea?

Answer 28

Epibenthic, crevice dwellers, under boulders, burrowers

Question 29

What do burrowers in the class Holothuroidea lack?

Answer 29

Some burrowers lack tube feet and burrow by peristaltic contractions using anchor shaped ossicles for traction (like chaetae in annelids)

Question 30

How do the class Holothuroidea feed?

Answer 30

Deposit and suspension feeders
Particles are trapped in mucus on buccal tentacles and transferred directly to the mouth

Question 31

How are the class Crinoidea described?

Answer 31

Ancient and primitive group of echinoderms
The only class where the oral surface is directed upwards
Include the attached sea lilies- very abundant in fossil record but only 80 modern species- restricted to deep water, can bend stalk and unroll arms when feeding

Question 32

What do the class Crinoidea contain?

Answer 32

Also include the feather stars-550 spp- free living but attached like a miniature sea lily as a juvenile- perch on cirri- can crawl and swim with arms

Question 33

How are the arms arranged in the class Crinoidea?

Answer 33

Arms are basically pentamerous but branched at the base- arms have additional lateral branches to give a large surface area- normally held in a funnel shape when feeding
Arms and pinnules have a ciliated ambulacral groove on oral surface

Question 34

How does suspension feeding work in the class Crinoidea?

Answer 34

When a particle contracts a tube foot it flicks it into a ambulacral groove where it is trapped by mucus and and transported by cilia to the mouth