Chaetognaths, Echinoderms, and

HemichordatesChapter 22

DeuterostomesDeuterostome

characteristics:Radial, indeterminate

cleavageFormation of the mouth

from a second openingEnterocoelous coelom

development

Chaetognaths are placed outside both protostome & deuterostome groups.

Phylum ChaetognathaThe arrow worms,

phylum Chaetognatha, are all marine, planktonic organisms.Some deuterostome

embryological characters.

Molecular works suggests they are protostomes.

Currently not considered to be part of either group.

Clade AmbulacrariaSuperphylum Ambulacraria contains two

deuterostome phylaEchinodermata and Hemichordata

Members share a three-part (tripartite) coelom, similar larval forms, and an axial complex (specialized metaneprhidium).

Xenoturbella is the sister taxon to Ambulacraria.Now considered to be a Phylum: Xenoturbellida

Phylum EchinodermataEchinoderms

include sea stars, brittle stars, sea urchins, crinoids, sea cucumbers.Entirely marine

Lack ability to osmoregulate.

Almost entirely benthic.

Nonsegmented.

Phylum Echinodermata

Five extant classes of echinoderms are currently recognized.

Phylum Echinodermata – Characteristics

Spiny endoskeleton of plates

Water vascular system

PedicellariaeDermal branchiae

(skin gills)Pentaradial symmetry

in adults

Phylum Echinodermata - SymmetryEchinoderms are

bilaterally symmetrical as larvae.This means their

ancestors were bilaterally symmetrical.

Phylum Echinodermata - SymmetryAs adults they show

secondary radial symmetry – pentaradial (5 parts).Perhaps an

adaptation for sessile living in early echinoderms.Crinoids

Phylum Echinodermata - SymmetryToday’s echinoderms are

mostly motile.Many are still radial.Some have again become

superficially bilateral (skeletal & organ systems still pentaradial).Sea cucumbers.A few sea urchins.

No well defined head or brain.

Phylum Echinodermata - Deuterostomes

Echinoderms have a true coelom with deuterostome development.Radial, indeterminate cleavage.Enterocoelous – the mesoderm lined coelom

develops from outpocketing of the primitive gut.Formation of the mouth at the end of the embryo

opposite the blastopore.

Water Vascular SystemEchinoderms have a

water vascular system derived from part of the coelom.

A system of canals and specialized tube feet that functions in:LocomotionFood gatheringRespirationExcretion

Water Vascular SystemThe water vascular

system opens to the outside through small pores in the madreporite.

Water Vascular SystemCanals of the water

vascular system lead to the tube feet.Tube feet may have

suckers, allowing the echinoderm to move while remaining firmly attached to the substrate – important in areas with lots of wave action.

EndoskeletonEchinoderms have an endoskeleton of

calcareous ossicles often with spines.Endoskeleton is covered by an epidermis.Some have a very substantial endoskeleton (sea

urchins), others have only a few scattered dermal ossicles (some sea cucumbers).

DevelopmentEggs (which may be brooded or laid as benthic egg

masses) hatch into bilateral, free-swimming larvae.The type of larva is specific to each echinoderm class.Class Asteroidea

BipinnariaBrachiolaria

Class OphiuroideaOphiopluteus

Class EchinoideaEchinopluteus

Class HolothuroideaAuricularia

Class CrinoideaDoliolaria

DevelopmentMetamorphosis involves a reorganization into

a radial juvenile.Left/right becomes oral/aboral.

Class AsteroideaClass Asteroidea

includes sea stars.

Common on rocky shores and coral reefs, some found on sandy substrates.

Class AsteroideaSea stars have

arms (rays) arranged around a central disc.

The body is flattened, flexible, and covered with a ciliated, pigmented epidermis.

Class AsteroideaThe mouth is on the

underside of the sea star.

Ambulacral grooves stretch out from the mouth along each ray.Tube feet border

each groove.

Class AsteroideaThe aboral surface

is often rough and spiny.

Around the base of each spine there are pincerlike pedicellariae that keep the surface free of debris and sometimes help with food capture.

Class AsteroideaSkin gills are soft

epidermis covered projections of the coelom that extend between ossicles and serve a respiratory function.

Class AsteroideaThe lower part of

the stomach can be everted through the mouth during feeding.

Class AsteroideaThe upper part of

the stomach connects to a pair of digestive glands (pyloric ceca) in each arm.

Class Asteroidea - FeedingMost sea stars

are carnivorous; feeding on molluscs, crustaceans, polychaetes, echinoderms, other inverts & sometimes small fish.

Class Asteroidea - Reproduction

Most sea stars have separate sexes with a pair of gonads in each ray.

Fertilization is external.

Class Asteroidea - Regeneration

Echinoderms can regenerate lost parts.

Sea stars can readily replace an arm if it is lost. This may take several months.They can also cast off an injured arm.

Class Asteroidea - Regeneration

Some species can even regenerate an entire individual from a broken off arm.Usually, a small piece of the central disc must be

included.Linckia can regenerate a whole new individual from a

broken arm with no central disc attached.

ConcentricycloideaThe two species of sea

daisies were described for the first time in 1986.

They are tiny (< 1 cm), have no arms and the tube feet are arranged around the periphery of the disc.

Once considered a separate class, they are highly derived sea stars.

Class OphiuroideaBrittle stars (Class

Ophiuroidea) are the largest group of echinoderms.

Abundant in all benthic marine environments – even the abyssal sea bottom.

Brittle stars have very slender arms.

Class OphiuroideaNo pedicillariae or

skin gills.

Madreporite is on the oral surface.

Tube feet have no suckers, their primary function is to aid in feeding.

Class OphiuroideaBrittle stars move using their arms rather

than tube feet.

http://youtu.be/BWOdssnzsMY http://youtu.be/4Texm2eTmSc

Class EchinoideaClass Echinoidea includes sea urchins and

sand dollars.

Class EchinoideaThe endoskeleton

is well developed in echinoids.

Dermal ossicles have become close-fitting plates that form the test.

http://www.jaxshells.org/test.htm

Class EchinoideaEchinoids lack arms, but still show the

pentamerous plan in the five ambulacral areas with pores in the test for the tube feet.

Class EchinoideaMost echinoids are “regular” having a hemispherical

shape, radial symmetry, and medium to long spines.

Regular urchins move using their tube feet with some help from spines.

Class Echinoidea“Irregular” echinoids include the sand dollars

and heart urchins that include some species that have become bilateral.

Spines are usually short and are used in locomotion.

Class EchinoideaSome urchins have very reduced tests, and

bright coloration.

The pedicellariae in these species contain painful toxins.

Class EchinoideaEchinoids live in all seas from the intertidal to

the deep sea.

Urchins usually prefer rocky substrate, while sand dollars and heart urchins like to burrow into sandy substrate.

Class EchinoideaEchinoids have a complex chewing mechanism

called Aristotle’s lantern.Teeth are attached here.

Sea urchins are usually omnivorous feeding mostly on algae.

Class EchinoideaSand dollars use

their short spines to move sand & its organic contents to the sides, the food particles drop between the spines, and ciliated tracts on the oral side carry the particles to the mouth.

Class HolothuroideaSea cucumbers (Class Holothuroidea) are

elongated along the oral/aboral axis.Bilateral

Ossicles are greatly reduced in most species.

Class Holothuroidea

The body wall is usually leathery with tiny ossicles embedded in it, but can be very thin.

Class HolothuroideaOral tentacles are

modified tube feet located around the mouth.

Food particles are gathered by the oral tentacles. Tentacles are put

into the pharynx one by one so food can be sucked off.

Class HolothuroideaSea cucumbers

move using ventral tube feet and waves of contraction along the muscular body wall.

Class HolothuroideaSea cucumbers have a

very unusual defense mechanism:They are able to cast out

part of their viscera.The lost parts

regenerate.Some have organs of

Cuvier that can be expelled in the direction of an enemy.These tubules become

long and sticky, sometimes containing toxins.

Class CrinoideaCrinoids include sea

lilies and feather stars.At metamorphosis,

juveniles become sessile and stalked.

Adults are free-moving in some species.

Long, many branched arms.

Class CrinoideaCrinoids use their

tube feet and mucus nets to feed on small organisms that are passed to their ciliated ambulacral grooves.

PhylogenyEchinoderms are probably derived from

bilateral ancestors.

Pentaradial symmetry may have been an adaptation to a sessile existence.

Some forms then become mobile.Some mobile forms are secondarily bilateral.

Phylum HemichordataHemichordates (acorn

worms) are marine animals that have gill slits and a rudimentary notochord – however, the notochord is not homologous with the notochord in vertebrates.

Phylum HemichordataVermiform bottom

dwellers, usually in shallow water.

Some are colonial living in secreted tubes.

Phylum HemichordataHemichordates are deuterostomes with radial

indeterminate cleavage and enterocoelous coelom development.

Larvae are similar to those of echinoderms.

Phylum HemichordataA tubular dorsal nerve

cord in the collar zone of acorn worms seems to be homologous to that in chordates.

Gill slits in the pharynx serve for filter feeding and secondarily for breathing – another characteristic found in chordates.

PhylogenyHemichordates share characteristics with

echinoderms:Early embryogenesisSimilar larvae

And Chordates:Gill slitsDorsal hollow nerve cord