Chapter 4 Animal Kingdom Part 4 (Phylum Mollusca , Echinodermata and Hemichordata) by TEACHING CARE online tuition and coaching classes
Animal Kingdom Part 4
(Phylum Mollusca, Echinodermata and Hemichordata)
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 It secretes the shell (if there is one). The mantle cavity contains gills or lungs
- The molluscs have bilateral symmetry, but snails are asymmetrical
- They are triploblastic, coelomate True coelom is reduced
and limited to kidney and gonads. The haemocoel is well developed.
- They have organ system grade of
- The molluscs are provided with one or two calcareous The shells may be external or internal.
- Respiration is carried out by the gills or pulmonary chambers.
- The digestive system is well developed with a characteristics rasping organ called a radula and a hepatopancreas.
- The circulatory system is of open type except in
- The excretion by organ of Bojanus or Kebers organ(pericardial gland).
- The nervous system is well
- The sensory organs are eyes, statocysts and osphradia. The osphradia are for tasting chemical nature of water
- Sexes are separate, or
- The development either direct or indirect involving veliger and trochophore larvae
Class 1. Aplacophora
- The head, mantle, foot, shell and nephridia are
- The body is covered with
- The digestive tract is straight with
- A mid dorsal longitudinal keel or crest present . Example : Chaetoderma
Class 2. Monoplacophora
- The shell is composed of a single
- The head is without eyes and 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.
- The foot is flat and
- The radula is well Example : Chiton
Class 4. Gastropoda
- These animals appear to be moving on their Hence the name gastropoda.
- Gastropods are asymmetrical.
- The shell is univalve and spirally
- The head is distinct with tentacles, eyes and a
- The foot is ventral and
- The buccal cavity is provided with a
- The circulatory system is
- The sexes are mostly
- 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
Class 6. Pelecypoda
- They are bilaterally symmetrical and laterally
- The shell is composed of two distinctive shell
- No distinct head.
- The sexes are separate or
- The development is indirect having a glochidium Example : Mytilus, Unio, Tredo.
Pearl oyster belongs to the class pelecypoda.
Class 7. Cephalopoda
- The foot is modified into arms and siphon (funnel).
- The shell may be either absent or rudimentary; it may be internal or
- The odonotophore with a radula is
- The ink glands for offence and defence are
- The sexes are
- The development is direct hence no metamorphosis and larval
Example : Nautilus, Loligo Sepia, Octopus
Dorsal view of a squid (loligo) in swimming position
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.
(1) Echinoderms are exclusively marine, free living, triplobalstic and coelomate animals.
- They are radially The radial symmetry is a secondary character in the adult echinoderms.
- They have organ system level of body
- They have a characteristics water vascular system with tube feet for locomotion, feeding and
- Circulatory system is of the open
- The sensory organs are poorly
- The excretory organs are
- 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.
- Development is indirect with bilaterally symmetrical
- 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
- They have great power of
General Anatomy of an Echinoderm (Starfish)
Class 1. Asteroidea
- It includes starfishes or sea
- Arms five and not sharply demarked from the central
- Tube feet with suckers in orally placed ambulacral
- Anus and madreporite
- Slow-creeping, predaceous and scavangerous. Examples : Astropecten, Oreaster (= Pentaceros),
Class 2. Ophiuroidea
- It includes Brittle-stars.
- Body star-like with arms sharply demarked off from the central
- Stomach sac-like without
- Ambulacral grooves absent or covered by ossicles; tube feet without
Examples : Ophiura, Ophiothrix, Ophioderma.
Oral view of the disk of a brittle star
Class 3. Echinoidea :
- The body is not divided into arms and is globular (sea urchins), or flattened disc-like (sea-cakes).
- Mouth present at lower end, covered by five strong, sharp teeth, forming a biting and chewing apparatus called “Aristotle’s Lantern”.
- Tube feet with
- Skin ossicles are fused together to form a rigid globular, disc like, or heart-shaped shell or test with movable
- Larval forms Pluteus and Echinopluteus. Examples – Sea urchins and sand
Oral view of the Atlantic sea urchin
Class 4. Holothuroidea
- Body long, cylindrical like a cucumber; elongated in oral aboral axis; without arms.
- Mouth at anterior and anus at posterior
- Mouth is surrounded by many hollow retractile
- Tube feet usually present; sucker-like.
- Skin leathery but without spines or
- Respiration and excretion by two long, highly branched tubes (known as respiratory tree) in
- Larval form Auricularia. Examples – Holothuria,
Sea cucumber (cucmaria frondosa)
Class 5. Crinoidea
- Body pentamerous; distinguished into a small, circular central disc and five or more (in multiples of five) long, branched arms radiating from the
- 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
- Mouth and anus both on oral
- Tube feet sucker like; restricted to central
- Some forms (sea lilies) permanently sessile and attached to sea bottom by a stalk; others (feather stars) free-swimming.
- Pedicellariae absent in
Examples – Sea lilies and Feather stars (Antedon)
A stalked feather star
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.
- Body divided into 3 regions: Proboscis, collar and
- 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
- Gill slits present but dorsal in position
- Ventral nerve chord
- Respiration by gills
- Circulatory system open
- Development mostly indirect with a free swimming larva called
Fig Balanoglossus (above)
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
- Natural pearl is a
(a) A molluscan secretion (b) Annelida secretion (c) An arthopoda secretion (d) An echinodermata secretion
- Mantle, foot and shell are the character of
(a) Nautilus (b) Echinus (c) Limulus (d) Euplectella
- Shell of mollusc is produced by its
(a) Radula (b) Thorax (c) Mantle (d) Abdomen
- Sea hare’ is
(a) Aplysia (b) Pila (c) Teredo (d) Septa
- Main function of pedicellariae in Asterias is
(a) Digestion (b) Excretion (c) Respiration (d) Capture of prey and removal of debris
- Aristotle’s lantern is a characteristic of the following class of echinodermata
(a) Echinoidea (b) Ophiuroidea (c) Holothuroidea (d) Asteroidea
- Antedon belongs to the class
(a) Crinoidea (b) Asteroidea (c) Ophiuroidea (d) Echinoidea
- Notochord is restricted to the anterior part of body proboscis in animals of which group
(a) Hemichordata (b) Urochordata (c) Cephalochordata (d) Chordata
- Presence of a skeleton of calcareous structure is found in all the members of which phylum
(a) Porifera (b) Echinodermata (c) Mollusca (d) Protozoa
- The term not applicable to echinodermata
(a) Madreporite (b) Podia (c) Marine (d) Bilateral symmetry
- The organ used for tasting chemical nature of water…….
- A living fossil and connecting link between Annelida and Mollusca is
- The glochidium larva is found in the phylum
- Ink glands found in mollusca are the characteristic of the class……
- The only non-chordate phylum having mesodermal endoskeleton made up of calcareous plates or ossicles is……….
- The free swimming characteristic larva of hemichordates is called as ……
- The bipinnaria larva belongs to the phylum…….
- What are pedicillariae?
- What is Aristotle’s Lantern
- Briefly explain about stomochord of hemichordates
Answers and Solutions
- Echinoderms are radially symmetrical
- The pincer like structures on the body surface in between the spines in echinoderms are called pedicellariae. The pedicillariae are used for cleaning the surface off
- The mouth in class echinoidea is covered by five strong, sharp teeth, forming a biting and chewing apparatus called “Aristotle’s Lantern”.
- The so called notochord in hemichordates is not a true notochord but it is a hollow outgrowth from the roof of buccal Therefore, it is also called as Buccal diverticulum or stomochord. Stomochord is confined to proboscis and non homologous with that of chordates