Lecture #13

Lecture #13

Phylum Echinodermata

Features common to Phylum Echinodermata

• 1. triploblastic• 2. pentaradial symmetry in adults; bilateral symmetry in larvae• 3. coelomate• 4. endoskeleton made of calcerous plates called ossicles• 5. water vascular system for locomotion• 6. complete digestive tract• 7. hemal system derived from the coelom – fluid filled canals,

function unknown• 8. nervous system consisting of a nerve net, a nerve ring and

radial nerves

Phylum Echinodermata• pentaradial symmetry

– body parts are arranged in fives or multiples of fives around an oral-aboral central axis

– adult form is pentaradial– larvae have bilateral symmetry

• internal/endoskeleton made of calcium carbonate– consisting of calcium carbonate plates called ossicles– skeleton can be modified to form articulating or fixed spines that project

from the body surface

Water vascular system• water-filled canals• extensions are called tube feet• includes a ring canal that surrounds the

mouth– ring canal opens to the outside through a

stone canal and a sieve-like pore called the madreporite

• five (or a multiple of five) radial canals branch from the ring canal and run down each arm or arch down the body wall toward the aboral end

• multiple lateral canals branch off of the radial canals and end in tube feet

madreporite

tube feet:https://www.youtube.com/watch?v=HPhAGyDceLo

-tube feet • extensions off the lateral canals • emerge to the outside through

openings between the ossicles• inside the body they end as a bulblike,

muscular ampulla• when the ampulla contracts- forces water

into the tube which then extends • outside the body - tube feet often

have suction cups at its distal end• function of tube feet: locomotion

Phylum Echinodermata• approximately 7,000 species• all are marine• living echinoderms are classified into 6 classes– 1. Asteroidea – sea stars– 2. Ophiuroidea – brittle stars and basket stars– 3. Echinodea – sea urchins and sand dollars– 4. Holothuroidea – sea cucumbers– 5. Crinoidea – sea lilies and feather stars– 6. Cocnentricycloidea – sea daises

• sea stars• about 1,500 species• live on hard substrates in marine

environments• five arms radiating from a central disc• mouth & tube feet are found on the

ventral side (oral surface) of this disc• endoskeleton comprised of ossicles• moveable spines project from the

ossicles – on the dorsal or aboral surface

• dermal branchiae – thin folds of the body wall found between ossicles – function in gas exchange and

excretion of metabolic wastes

Class Asteroidea

• in some species – the aboral surface has pincer-like appendages called pedicelleriae

• series of ossicles on the oral surface of each arm form an ambulacral groove– each groove houses a radial canal– paired rows of tube feet protrude through the body wall

on either side of the groove

Class Asteroidea

ambulacral groove

• locomotion: tube feet move in a stepping motion– contraction of ampulla moves fluid into the tube foot– alternate extension, attachment, and contraction of these feet move the sea star

across its substrate– suction discs on the tube feet allow effective adhesion in strong currents

Class Asteroidea

• nervous system: – consists of a nerve ring

that encircles the mouth– plus radial nerves down

each arm or running down the body wall toward the aboral end• responsible for coordination

of the tube feet– body wall also houses a

nerve net– sensory receptors are

distributed over the surface of the body and the tube feet

Class Asteroidea

• digestion and feeding:– mouth surrounded by moveable oral

spines – mouth opens to a short esophagus – esophagus leads into a larger cardiac

stomach (oral stomach) for the receipt of food

– then into a smaller pyloric stomach (aboral stomach)

– pyloric stomach leads into digestive glands - called pyloric cecae – 2 per arm• secretory and absorptive functions

– large, undigested materials are expelled out through the mouth NOT the anus

Class Asteroidea

mouth

• sea stars feed on bivalves by prying open the shell• tube feet attach to the outside of the shell and forces the valves apart• once open slightly - everts the cardiac portion of the stomach into the mollusc• releases digestive enzymes and partial digestion weakens the bivalves adductor

muscles - the sea star then pulls the bivalve open completely• takes in the partially digested mollusc for continued digestion in the cardiac

stomach

https://www.youtube.com/watch?v=2DFXGafpGkQ

• reproduction:– dioecious– two gonads present in each arm – open via gonopores at the base of the

arm– external fertilization– gamete release is timed with pheromone release that induces other sea

stars to spawn – increases chances of fertilization– embryology has been studied extensively – easy to spawn and fertilize in

the lab• form a bilateral embryo = bipinnaria larva• float around in the plankton• development of radial arms results in a branchiolaria larva – develops into a

juvenile sea star– regenerative powers are remarkable – can regenerate entire radial arms

Class Echinoidea• sea urchins and sand dollars• about 1,000 species• nearly all marine• sea urchins specialized for living on hard

substrates – wedging themselves into cracks and crevices

• sand dollars – burrow into sand or mud– use their tube feet to catch organic matter settling on

them or passing over them

Class Echinoidea

• sea urchin:– skeleton is called a test– 10 closely fitted plates that arch between

oral and aboral ends– five rows of ambulacral plates have

openings for tube feet– alternate with 5 rows of interambulacral

plates with spines– pedicelleriae & moveable spines - for prey

capture and cleaning– water vascular system is similar to sea stars– radial canals run along the inner wall –

between the oral and aboral ends

some spines are hollow and contain venom to kill prey

• feeding and digestion:– feed on algae, bryozoans, corals and other

echinoderms– oral end with a mouth and oral tube feet –

faces toward the substrate– possess a chewing apparatus called

Aristotle’s lantern• can be projected from the mouth• cuts food into small pieces for ingestion

mouth

lantern: https://www.youtube.com/watch?v=9TeZGi4-TOA

• gas exchange: diffusion across the tube feet• similar nervous system to the sea stars• reproduction and development:

– dioecious– five gonads along the body wall of

interambulacral plates– the gonads open at ossicles found at the aboral

end - called genital plates – external fertilization

anus

genital plate withgonopore

Class Holothuroidea• sea cucumbers• about 1,500 species• found at all depths of the ocean• have no radial arms• elongated along the oral-aboral axis• lie on one side which is flattened – called the ventral side• thick and muscular body wall

– lacks spines or pedicelleriae• outer epidermis covering a dermis with embedded ossicles• 10 larger ossicles form a ring canal that surrounds the oral end • below the dermis – layer of circular muscle overlying longitudinal

muscles

http://webs.lander.edu/rsfox/invertebrates/cucumaria.html

• water-vascular system is filled with coelomic fluid• ring canal – surrounds the oral end of the digestive

tract– 5 radial canals arise from the ring canal – give rise to

tube feet that run from oral to aboral poles• ventral side has 3 rows of tube feet used for

attachment• dorsal surface has two rows of tube feet – may be

absent in some• tube feet surrounding the mouth – can elongate and

are often referred to as tentacles• internal madreporite

– connected to the ring canal via a stone canal

Class Holothuroidea

tube feet

• digestive system – ingest matter using their tentacles– mouth leads to a stomach plus a long, looped

intestine – then leads to a rectum and an anus– tentacles are thrust into their mouths to “wipe off”

the food trapped in mucus– extracellular digestion in the intestine– hemal system for food distribution??

• nervous system – similar to other echinoderms

Class Holothuroidea

• Polian vesicles– maintain fluid pressure and volume in the water vascular

system• respiratory trees for gas exchange

– attach at the rectum and branch throughout the body

– pumping action of the rectum circulates water through these trees

– water is drawn into these trees – gases diffuse into the colemic fluid surrounding the trees

– fluid is circulated through the coelom for distribution of respiratory gases, wastes and nutrients

Class Holothuroidea