M15 Echinoderms

Question 1

Echinoderms (+ classes)

Answer 1

Make up 4.39% of Kingdom Animalia. 7,300 species. Classes: Ophiuroidea (brittle star), Asteroidea (sea stars), Holothuroidea (sea cucumbers), Echnioidea (sea urchins), Crinoidea (sea lilies, feather stars)

Question 2

Typically

Answer 2

All marine, no heads
-Adults have radial symmetry, pentamerous, larvae are bilateral
-Calcareous mesodermal skeleton (ossicles, plates, often with tubercles or spines)
-Presence of water vascular system
-Tube feet for locomotion or feeding
-Decentralised nervous system (nerve net)

Question 3

Class Ophiuroidea

Answer 3

2000 species (brittle stars)
-Free living, distinct skeleton
-Long arms for walking and swimming (distinct from body)
-Mouth on lower surface
-Central body is small in a disk shape, large stomach but without an anus or gut
-Tube feet are used for suspension feeding

Question 4

Class Asteroidea

Answer 4

1900 species
-5-40 arms, covered in spines, may be modified into pedicellariae (pincer like structures)
-Aboral surface (anus), madreporite is for water entry for water vascular system using ampullae connected to tube feet on oral surface, water flows through muscular contractions. Digestive parts/guts run from mouth to anus (on top if present)
-Oral surface (mouth downward), tube feet, often suckered, extend from ambulacral grooves in sea stars’ oral (bottom) surface for locomotion.
-Some tube feet are modified for gas exchange

Question 5

Ossicles

Answer 5

Small calcareous elements embedded in the dermis of the body wall of echinoderms. Form part of the exoskeleton (rigidity and protection). Found in different forms/arrangements.

Question 6

Class Echinoidea

Answer 6

1000 species
-Spherical or secondarily flattened with arms incorporated into the body
-Calcareous ossicles fused into solid skeleton (test), spines + tube feet for movement and clinging to hard surfaces
-Coiled digestive system which consists basically of a tube joining the lower mouth to the anus on the upper surface
-Feature a unique feeding apparatus called ‘Aristotle’s lantern’ (5-sided beak) for grazers, scavengers of carnivores.

Question 7

Aristotle’s lantern

Answer 7

Intricate jaw apparatus = five jaws made of calcium plates, connected by muscles. Use this to scrape algae off rocks.

Question 8

Tube feet

Answer 8

Use them as eyes, possibly also have light receptors (red) and are found across the entire body.

Question 9

Class Holothurians

Answer 9

1700 species (sea cucumbers)
-Pentamerous and bilateral symmetry
-Arms incorporated in body
-Gas exchange via respiratory trees, breathe by drawing in water through anus and the expelling it
-Calcareous skeleton is reduced (ossicles)
-Locomotion is slow with tube feet, deep sea forms

Question 10

Gut generation in sea cucumbers

Answer 10

They can completely disregard most of their internal organs and then rapidly regrow them using specialised cells, called evisceration which refers to just the intestine or whole anterior end. They also sue the guts to fight often bigger predators.

Question 11

Class Crinoidea

Answer 11

620 species
-Have ancestral body posture (echinoderms), upward mouth and tube feet for feeding, not locomotion.
-Five arms, but often divided repeatedly
-50% species live 0-20m deep.