Animal Kingdom Part 4

(Phylum Mollusca, Echinodermata and Hemichordata)

Phylum Mollusca

The mollusks are soft bodied animals. (L., mollis or molluscum = soft)The body is unsegmented and enclosed within a skin fold (mantle/pallium). The mantle secretes a calcareous shell. They have about 80,000 living and 35,000 extinct species known. Aquatic or terrestrial. Malacology or Conchology is the study of molluscs and their shells.

Body Plan of molluscs:-

• head: contains sensory organs
• foot: Muscular organ used for locomotion
• visceral mass: contains digestive, reproductive, circulatory organs
• mantle: skin of the dorsal body wall. It secretes the shell (if there is one). The mantle cavity contains gills or lungs

Source:http://infusion.allconet.org/webquest/PhylumMollusca.html

General characters:

1. The molluscs have bilateral symmetry, but snails are asymmetrical
2. They are triploblastic, coelomate animals. True coelom is reduced

and limited to kidney and gonads. The haemocoel is well developed.

3. They have organ system grade of organization.
4. The molluscs are provided with one or two calcareous shells. The shells may be external or internal.
5. Respiration is carried out by the gills or pulmonary chambers.
6. The digestive system is well developed with a characteristics rasping organ called a radula and a hepatopancreas.
7. The circulatory system is of open type except in cephalopods.
8. The excretion by organ of Bojanus or Kebers organ(pericardial gland).

8. The nervous system is well developed.
9. The sensory organs are eyes, statocysts and osphradia. The osphradia are for tasting chemical nature of water
10. Sexes are separate, or hermaphrodites.
11. The development either direct or indirect involving veliger and trochophore larvae

Classification:

Class 1. Aplacophora

1. The head, mantle, foot, shell and nephridia are absent.
2. The body is covered with cuticle.
3. The digestive tract is straight with radula.
4. A mid dorsal longitudinal keel or crest present . Example : Chaetoderma

Class 2. Monoplacophora

1. The shell is composed of a single valve.
2. The head is without eyes and tentacles. Example : Neopilina galatheae

Neopilina is a living fossil and connecting link between Annelida and Mollusca.

Class 3. Polyplacophora

(1) The shell is composed of a longitudinal series of 8 shell plates.

3. The foot is flat and ventral.
4. The radula is well developed. Example : Chiton

Chiton

Source: http://www.thinkquest.org

Class 4. Gastropoda

1. These animals appear to be moving on their stomach. Hence the name gastropoda.
2. Gastropods are asymmetrical.
3. The shell is univalve and spirally coiled.
4. The head is distinct with tentacles, eyes and a mouth.
5. The foot is ventral and muscular.
6. The buccal cavity is provided with a radula.

8. The circulatory system is open.
9. The sexes are mostly separate.
10. The development includes veliger and trochophore larvae. Examples : Haliotis, Cyprea (Cowrie) Pila (apple snail), Murex (rock shell) Aplysia (sea hare), Doris (Sea Lemon)

Class 5. Scaphopoda

(1) They have a tubular shell opening at both ends. (2)The foot is boat shaped

Examples : Dentalium

Dentalium is commonly called tusk shells.

A scaphopod, or tooth shell, burried in the sand

Source: http://www.thinkquest.org

Class 6. Pelecypoda

1. They are bilaterally symmetrical and laterally compressed.
2. The shell is composed of two distinctive shell plates.

4. No distinct head.
5. The sexes are separate or united.
6. The development is indirect having a glochidium larva. Example : Mytilus, Unio, Tredo.

Pearl oyster belongs to the class pelecypoda.

Class 7. Cephalopoda

1. The foot is modified into arms and siphon (funnel).
2. The shell may be either absent or rudimentary; it may be internal or external.
3. The odonotophore with a radula is present.
4. The ink glands for offence and defence are present.

7. The sexes are separate.
8. The development is direct hence no metamorphosis and larval

stage.

Example : Nautilus, Loligo Sepia, Octopus

Dorsal view of a squid (loligo) in swimming position

Source: http://www.thinkquest.org

Phylum Echinodermata

The term “Echinodermata” means spiny skin (Gr., echinos = spiny + dermatos = skin). They have a tough, leathery, spiny skin and a mesodermal endoskeleton made up of calcareous plates or ossicles. The echinoderms belong to deuterostome line of evolution. Their larva have the basic bilateral symmetry but the adults have pentamerous radial symmetry. About 6,000 living species are known.

General characters:

(1) Echinoderms are exclusively marine, free living, triplobalstic and coelomate animals.

3. They are radially symmetrical. The radial symmetry is a secondary character in the adult echinoderms.
4. They have organ system level of body organization.
5. They have a characteristics water vascular system with tube feet for locomotion, feeding and respiration.

6. Circulatory system is of the open type.
7. The sensory organs are poorly developed.
8. The excretory organs are absent.
9. They have pincer like structures on the body surface in between the spines called pedicellariae. The pedicillariae are used for cleaning the surface off debris.
10. Development is indirect with bilaterally symmetrical larva.
11. Respiration by dermal branchiae or papulae in star fishes, peristomial gills in sea urchins, genital bursae in brittle stars, and cloacal respiratory tree in holothurians
12. They have great power of regeneration.

General Anatomy of an Echinoderm (Starfish)

Source: http://www.thinkquest.org

Classification:

Class 1. Asteroidea

1. It includes starfishes or sea stars.
2. Arms five and not sharply demarked from the central disc.
3. Tube feet with suckers in orally placed ambulacral grooves.
4. Anus and madreporite aboral.
5. Pedicellariae present.
6. Slow-creeping, predaceous and scavangerous. Examples : Astropecten, Oreaster (= Pentaceros), Asterias.

Class 2. Ophiuroidea

1. It includes Brittle-stars.
2. Body star-like with arms sharply demarked off from the central disc.
3. Pedicellariae absent.
4. Stomach sac-like without anus.
5. Ambulacral grooves absent or covered by ossicles; tube feet without suckers.

6. Madreporite oral.

Examples : Ophiura, Ophiothrix, Ophioderma.

Oral view of the disk of a brittle star

Source: http://www.thinkquest.org

Class 3. Echinoidea :

1. The body is not divided into arms and is globular (sea urchins), or flattened disc-like (sea-cakes).
2. Mouth present at lower end, covered by five strong, sharp teeth, forming a biting and chewing apparatus called “Aristotle's Lantern”.
3. Tube feet with suckers.
4. Skin ossicles are fused together to form a rigid globular, disc like, or heart-shaped shell or test with movable spines.
5. Pedicellariae present.
6. Anus present.
7. Larval forms Pluteus and Echinopluteus. Examples – Sea urchins and sand dollars.

Oral view of the Atlantic sea urchin

Source: http://www.thinkquest.org

Class 4. Holothuroidea

1. Body long, cylindrical like a cucumber; elongated in oral aboral axis; without arms.
2. Mouth at anterior and anus at posterior ends.
3. Mouth is surrounded by many hollow retractile tentacles.
4. Tube feet usually present; sucker-like.
5. Skin leathery but without spines or pedicellariae.
6. Respiration and excretion by two long, highly branched tubes (known as respiratory tree) in cloaca.
7. Larval form Auricularia. Examples – Holothuria, Cucumaria.

Sea cucumber (cucmaria frondosa)

Source: http://www.thinkquest.org

Class 5. Crinoidea

1. Body pentamerous; distinguished into a small, circular central disc and five or more (in multiples of five) long, branched arms radiating from the disc.
2. Disc enclosed in a cup shaped calyx of calcareous plates; calyx attached to a substratum with the help of a stalk or simply by its aboral surface.
3. Mouth and anus both on oral surface.
4. Tube feet sucker like; restricted to central disc.
5. Some forms (sea lilies) permanently sessile and attached to sea bottom by a stalk; others (feather stars) free-swimming.
6. Pedicellariae absent in skin.

Examples – Sea lilies and Feather stars (Antedon)

A stalked feather star

Source: http://www.thinkquest.org

Phylum Hemichordata :

(Gr. hemi, half; chorde, cord)

The hemichordates were earlier considered as a subphylum of Chordata but now it is undoubtedly placed as a phylum under non chordates.

General Characters:

1. Body divided into 3 regions: Proboscis, collar and trunk.
2. The so called notochord in hemichordates is not a true notochord but it is a hollow outgrowth from the roof of buccal cavity. Therefore, it is also called as Buccal diverticulum or stomochord. Stomochord is confined to proboscis and non homologous with that of chordates.
3. Gill slits present but dorsal in position
4. Ventral nerve chord
5. Respiration by gills
6. Circulatory system open
7. Development mostly indirect with a free swimming larva called

Tornaria larva

Fig Balanoglossus (above)

CLASSIFICATIPON:

Class 1. Enteropneusta : (Gr. enteron, gut; pneustos, breathed). Body large and worm-like. Gill slits numerous. Intstine straight. Acorn or togue worms. 70 species. Balanoglossus, Saccoglossus.

Class 2. Pterobranchia : (Gr. pteron, feather; branchion, gill). Body small and compact. Gill-slits one pair or none. Intstine U–shaped.

Pterobranchs. 20 species. Cephalodiscus, Rhabdopleura.

Practice test Paper

1. Natural pearl is a

(a) A molluscan secretion (b) Annelida secretion (c) An arthopoda secretion (d) An echinodermata secretion

2. Mantle, foot and shell are the character of

(a) Nautilus (b) Echinus (c) Limulus (d) Euplectella

3. Shell of mollusc is produced by its

(a) Radula (b) Thorax (c) Mantle (d) Abdomen

4. Sea hare' is

(a) Aplysia (b) Pila (c) Teredo (d) Septa

5. Main function of pedicellariae in Asterias is

(a) Digestion (b) Excretion (c) Respiration (d) Capture of prey and removal of debris

6. Aristotle's lantern is a characteristic of the following class of echinodermata

(a) Echinoidea (b) Ophiuroidea (c) Holothuroidea (d) Asteroidea

7. Antedon belongs to the class

(a) Crinoidea (b) Asteroidea (c) Ophiuroidea (d) Echinoidea

8. Notochord is restricted to the anterior part of body proboscis in animals of which group

(a) Hemichordata (b) Urochordata (c) Cephalochordata (d) Chordata

9. Presence of a skeleton of calcareous structure is found in all the members of which phylum

(a) Porifera (b) Echinodermata (c) Mollusca (d) Protozoa

10. The term not applicable to echinodermata

(a) Madreporite (b) Podia (c) Marine (d) Bilateral symmetry

11. The organ used for tasting chemical nature of water…….
12. A living fossil and connecting link between Annelida and Mollusca is

………

13. The glochidium larva is found in the phylum

14. Ink glands found in mollusca are the characteristic of the class……
15. The only non-chordate phylum having mesodermal endoskeleton made up of calcareous plates or ossicles is……….
16. The free swimming characteristic larva of hemichordates is called as ……
17. The bipinnaria larva belongs to the phylum…….
18. What are pedicillariae?
19. What is Aristotle’s Lantern
20. Briefly explain about stomochord of hemichordates

Answers and Solutions

1. a
2. a
3. c
4. a
5. d
6. a
7. a
8. a
9. c
10. d.   Echinoderms are radially symmetrical