PHYLUM - ANNELIDA
The phylum annelida includes the segmented worms, such as earthworm and leeches.
Characteristic of Annelida
1. Body Form :- The body is soft, elongated, cylindrical or flattened. It is divided into segments or metamers by ring-like grooves, the annuli. This character is given by the phylum its name.
2. Symmetry :- The symmetry is bilateral in annelids.
3. Germ layers :- The segmented worms are triploblastic.
4. Level of organisation :-The segmented worms have organ system level of organisation.
5. Head:- The anterior end of the body often forms a distinct head with sense organs in some cases.
6. Appendages :-Many forms have simple, unjointed,locomotory appendages, the parapodia, on the side of the body.
7. Body wall:- The body wall consists of thin, most cuticles, single layered epidermis and well developed musculature. The cuticle is albuminoid rather than chitinoid, and allows free exchange of gases. The musculature is differentiated into circular and longitudinal layers of muscle fibers. All muscle fibers are smooth.
8. Body cavity :-The body cavity is a true coelom, as it is lined by a mesodermal coelomic epithelium. It is schizocoely. It is divided by transverse septa into compartments. It is filled with coelomic fluid that contains cells.
9. Skeleton :- There is no mineralised skeleton. Fluids filling the coelom serve as a hydrostatic skeleton.
10. Digestive Tract:- The digestive tract is complete, straight and extended through the entire body. The gut has both circular and longitudinal muscle.
11. Circulatory System :- A closed circulatory system has appeared. Some blood vessels are enlarged to act as pumping hearts. The blood is red with haemoglobin dissolved in the plasma. It has amoeboid corpuscles only.
12. Excretory system :- The excretory system consists of coiled tubules, the metanephridia. The letter also helps in the maintenance of water balance of the body (osmoregulation). Excretory matter is ammonia in aquatic form (Ammonotelism) and urea in landforms (ureotelism) .
13. Nervous system :-The nervous system includes a circumentric nerve ring and solid, double, mid ventral, nerve cord with ganglia (ganglion is an aggregation of nerve cell bodies). Sense organs (tentacles, palps, eyes) may be present.
14. Reproduction :- The sexes may be separate or united. Asexual reproduction by budding or fission occurs in some cases.
15. Development :- Life history may include a trochophore larva in some cases.
UNIQUE FEATURES
The annelids have the following unique features -
Metameric segmentation.
Nephridia for excretion and osmoregulation.
Closed circulatory system with respiratory pigment dissolved in the plasma.
ADVANTAGE OVER ROUNDWORMS
The annelids show advantage over the roundworms in having -
Metameric segmentation
Head, appendages and respiratory organ (gills) In some cases
True coelom
Circulatory systems often have red blood.
Circular and longitudinal muscle in both body wall and gut wall.
CLASSIFICATION
The phylum annelida is divided Into three classes :-
Polychaeta
Oligochaeta
Hirudinea
Class1. Polychaeta
The polychaeta have following characters -
Body bears a distinct head with sense organs (eyes, tentacle, palps), and unjointed locomotory appendages called paralodia.
Setae are long, numerous and occur in the parapodia.
Saxes are separate, Gonads are temporary, developing in breeding season only.
Fertilisation is generally external.
Development includes a larval stage, called trochophore.
Asexual reproduction by budding occurs in some cases.
Polychaetes are mostly marine.
Examples : Nereis -the sandworm, Aphrodite - the sea mouse
class 2. Oligochaeta
Thank oligochaeta have following characters -
Body lacks head and parapodia.
Setae are small and fewer.
Clitellum is present.
Sexes are united.
Fertilization occurs in the ootheca.
Development is direct.
Asexual reproduction is common in freshwater forms.
Oligochaetes are terrestrial or fresh-water annelids.
Examples : pheretima - The earthworm, Tubifex -
bloodworm
Class 3.Hiridinea
The hirudinea have following characters -
Body lacks head and parapodia.
Setae are also absent.
There is an anterior sucker and often also a posterior sicker for locomotion and feeding.
Ceiling is greatly reduced by the formation of peculiar connective tissue called botryoidal tissue.
Sexes are united.
Development is direct.
Example : Hirudinaria
PHYLUM - MOLLUSCA
The phylum Mollusca includes the mussels, oysters, snail, slugs, squids and cuttlefish.
They possess the following characters-
1. Body form:- The body has a variety of shapes and is usually unsegmented. Neopilina is, however, a segmented mollusk.
2. Symmetry :- The symmetry is generally bilateral in mollusks. Some are secondarily asymmetrical (snail) due to torsion (twisting) during growth.
3. Germ layers :- The mollusks have organ system level of organisation.
4. Body parts:- The body is usually differentiated into three regions : Anterior head with sense organs, dorsal visceral mass containing organ system and ventral foot for locomotion.
5. Body wall :- Epidermis is 1-layered and usually ciliated. Muscles are unscripted and occur in bundles.
6. Mantle (pallium) :-A thin fleshy fold or outgrown dorsal body wall more or less covers the body. This fold is called the mantle or pallium. It encloses a space, the mantle, or pallial, cavity, between itself and the body.
8. Shell:- The mantle usually secrets an external slimy.
9. Body cavity :- Coelom is greatly reduced. It is represented by cavities in the pericardium, kidneys and gonads.
Space amongst the viscera contains blood from haemocoel.
10. Digestive tract :- The digestive tract is complete. Buccal cavities often contain a rasping organ, the redula, with transverse rows of teeth. Annus opens into the mantle.
11. Respiration :- Respiration usually takes place by gills, called the ctenidia, located in the mantle cavity, but may occur by body surface, mantle (Dentalium) or pila.
12. Circulatory system :- The circulatory system is mostly open. It includes a dorsal pulsatile heart and a few arteries that open into sinuses. Blood often has a copper containing, blue respiratory pigment called haemocyanin. The cephalopods have a closed circulatory system.
13. Excretory system :- The excretory includes 1 or 2 pairs of socks like a kidney, which open into the mantle cavity. The excretory matter is ammonia or uric acid ( Ammonotelism or uricotelism)
14. Nervous system :- The nervous system typically Comprises three paired ganglia : cerebral above the mouth, pedal in the foot, and visceral in the visceral mass, interconnected by commissures and connectives. A commissure joins similar ganglia. A connective links dissimilar ganglia. Sense organs included eyes, statocysts for equilibrium,and osphradia for testing the chemical and physical nature of water.
15. Reproduction :-Saxes are usually separate. Gonads have ducts. Fertilisation may be external or internal. Asexual reproduction is lacking.
16. Development :- Life history may be direct or with a larva named glochidium or veliger.
UNIQUE FEATURE
The mollusks show the following unique features -
Three body regions : head, visceral mass and foot.
A glandular fold, the mantle, over the body.
Mantle cavity with anal, excretory and genital aperatus in it.
Calcareous shell around the body in most cases.
A rasping organ, the radula, in the buccal cavity.
CLASSIFICATION
The phylum mollusca is divided into six classes :
MONOPLACOPHORA
AMPHINEURA
SCAPHOPODA
GASTROPODS
PELECYPODA
CEPHALOPODA
Class 1. Monoplacophora
The monoplacophora are all Marin. They have following characters -
Head bears tentacles.
Foot is disc-like with a flat, creeping sole.
Shell is Spoon or cup-shaped.
Visceral mass is segmented.
Example : Neopilina
Neoplina is a good connecting link between annelids and mollusks.
Class. 2 Amphineura
The Amphineura are all Marine. They have following characters -
Head is reduced and without eyes and tentacles.
Foot may be large, reduced or absent.
Shell may be absent or consist of a row of plates.
Example : Chiton
Class 3. Scaphopoda
The scaphopoda are all Marine . They have following characters -
Head is rudimentary and lacks eyes and true tentacles.
Foot is comical and used for digging.
Shell is tubular, open at either end.
Respiration occurs by mantle.
Example : dentalium - tusk shell or tooth shell
class 4. Gastropoda
The gastropods are terrestrial, freshwater as well as marine. They have following characters -
Head is present. It bears eyes and tentacles.
Foot is large and flat.
Shell is often spirally coiled.
Gills are pectinate.
Symmetrical embryo grows into an asymmetrical adult due to twisting (torsion) of the visceral mass during development.
The mouth and annus lie on the same side.
Examples : pila - the apple snail,
Cypraea - the cowrie,
Limax -the grey slug,
Helix -the garden snail,
class 5. Pelecypoda
The pelecypoda may be freshwater or marine. They have the following characters -
Head is absent.
Foot is wedge-shaped for burrowing.
Shell consists of two valves Movably hinged dorsally.
Gills are plate-like.
Unio
Examples: Unio - the fresh water mussel,
Ostrea - The edible oysters.
Nautilus - The sea- mussel
Class 6. Cephalopoda-
The cephalopods are all marine.They have the following characters -
Head and foot are combined together, hence cephalopoda.
Head is prominent and bears large eyes that resemble those of vertebrates.
Octopus
Shell is often internal and reduced. It may be external (Nautilus) or absent (octopus).
Gills are plume -like.
Circulatory system is closed.
Cuttlefish
Examples : Sepia - the cuttlefish, Loligo -the squid,
Octopus - the devil fish .
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