PENTACEROSSea Pentagon or Starfish

By

Pranjal Gupta, BSc(H) Zoology, Ramjas College

1

Phylum-Echinodermata

Key features: Echinodermata means- “echinos – "hedgehog"

and derma – "skin”, i.e Spiny skinned organisms

Have deutrostomic evolutionary line.

Possess a true coelom called enterocoelom.

Have mesodermal skeleton made of calareousossicles.

Triploblastic and radially symmetrical. 2

3

Subphylum-Eleutherozoa

Key features:

Mobile animals with the mouth directed towards

the substrate.

Non pedicellate and free living.

All living echinoderms except Crinodea( Sea

feathers) belong here.

4

Class-Asteriodea

Key features:

Star shaped echinoderms hence called starfish.

Have a central disc and fused arms(5 or more)

Have tube feet operated by hydraulic system.

Shows the property of autotomy.

Pedicellariae present.

Anus and madreporite aboral.

Larval stages include Bippinaria and Brachiolaria. 5

Order-Phanerozonia

Key features:

Body with marginal plates, with papulae(dermal

branchiae or skin gill) on aboral surface.

Pedicellariae sessile, not crossed.

Tube feet without suckers.

Mostly burrowers in soft bottom.

6

Genus- Pentaceros

Key features:

Large central disc, five arms short and tapering.

Aboral surface concave with definite arranged spines.

Reticulate aboral skeleton, enclosing most of the body.

Upper marginal plates smaller than ventral one.

Pedicellariae small and valvate type.

Five ambulacral grooves with two double rows of tube feet.

7

GENERAL FEATURES OF STARFISH

8

Pedicellariae

Madreporite

Stone canal

Ring canal

Radial canal

Arm

Cardiac stomach

Ambulacral ridge

Ampulla

Gonads

Pyloric duct

Pyloric ceca

Lateral canal

Dissection through aboral surface

9

LOCOMOTION

Tube feet

Tube feet are the many small tubular projections found on the oral

surface of starfish.

10

Overview of tube feet:

1. Tube feet function in locomotion, feeding and

respiration.

2. The tube feet in a sea star are arranged in

grooves along the arms.

3. They operate through hydraulic pressure.

4. A sea star that is overturned simply turns one

arm over and attaches it to a solid surface, and

levers itself the right way up.

11

T.S OF THE ARM OF PENTACEROS

Diagrammatical

representationT.S through microscope

Tube feet consist of two parts: ampulla and podia. Ampulla contain both circular

muscles and longitudinal muscle, whereas the podia contain the latter only. Thus

the podia use suction to attach to the substratum.

12

HYDRAULIC SYSTEM:

13

FEEDING MECHANISM

14

To grab hold of its prey, starfish first directs its mouth towards it attaches itself to the prey via the tube feet near the mouth then through its strong arms gets hold of the prey completely. For a clam it breaks open the bivalve shell through suction mechanism.

Once the clam is open, it ejects its cardiac stomach placing it over the digestible parts of its prey. The cardiac and the pyloric stomach then partially digests what it can, producing a chowder-like slurry that is then drawn back into the starfishes’ ten digestive glands.

15

Starfish’s mouth Starfish devouring a clam

Pyloric stomach visible

16

AMBULACRAL OR WATER VASCULAR SYSTEM

17

SEXUAL REPRODUCTION

18

DEVELOPMENT OF PENTACEROS

Gastrulation

Bippinaria larva

Brachiolaria larvaJuvenile starfish

metamorphosis

Adult pentaceros

Early Bippinaria larva

19

OTHER BIOLOGICAL SYSTEMS OF STARFISH Respiration occurs by dermal branchae or papulae.

Excretion(NH3) by diffusion via gills or derminal branchia.

Circulatory system open type and includes the haemal and

perihaemal system.

Asexual reproduction- fission of the central discs or by

regeneration(autotomy).

Nervous system with a nerve ring around the mouth and a

radial nerve in the ambulacral region of each arm.

Tiedemann's bodies and polian vescicles are pouches on the

ring canal whose function may be osmoregulation or

hydraulic regulation within the water vascular system.

Sensory organs-

1. Pedicellariae sensitive to touch.

2. Tube feet sensitive towards external factors and

orientation.

3. eyespots at the ends of the arms, each one made of 80–

200 simple ocelli covered with cuticle.20

DISTRIBUTION AND ECOLOGICAL ASPECTS OF PENTACEROS

Distribution- It is found in U.K., U.S.A., India, Panama and South America.

Ecological status- Pentaceros is a threat to the pearl industry since it generally devours on the pearl producing oysters( Ostrea ).

Sometimes it is also called the cushioned star.

Usually found in calm shallow waters.

It has been overharvested for souvenirs and the aquarium trade.

21

22