Animal Kingdom : Phylum Arthropoda and Mollusca Notes for NEET 2020

Check Phylum Arthropoda and Mollusca Notes for NEET 2020 exam! In medical exams like NEET, AIIMS, and JIPMER there are many questions asked from the Animal Kingdom. This is one of the important and Conceptual Unit. Here we are sharing detailed notes on Phylum Arthropoda and Mollusca. So Let's begin with a brief introduction of Animal Kingdom and its Classification.

Introduction:

The Animal Kingdom represents the heterotrophic, eukaryotic and multicellular organisms. As per the presence or absence of the notochord, the Kingdom Animalia can be broadly classified as:

1. Non-Chordata: Animals that do not have the notochord.
2. Chordata: Animals that have the notochord persistent or at some phase of their life.

The phyla Arthropoda and Mollusca represent the non-chordates. These have been discussed in details in the following heads.

PHYLUM ARTHROPODA

GENERAL FEATURES:

• The phylum Arthropoda is the largest phylum containing more than 8,00,000 species.
• The Arthropods can be seen in a large variety of habitats like aerial, aquatic, terrestrial, parasitic etc. These are known to occur 30,000 ft below the sea-level.
• Due to the presence of paired and jointed appendages either in all the segments or in some segments, these are called Arthropods (arthros-jointed; poda-feet)
• These animals are bilaterally symmetrical because of cephalization.
• The body is segmented. Each segment is not the repetition of the other segment (as seen in Annelida) but many segments fuse to form a particular structure which is characterised to perform specific functions. Such segments are called tagmata.
• These animals have a true coelom.
• Their body is provided with a tough covering of chitinous exoskeleton in the form of the plates called sclerites.

DIGESTIVE SYSTEM

The digestive system is complete with the anterior mouth and the posterior anus. It can be studied under the following heads:

1. Foregut or Stomodaeum: The mouth, pharynx, oesophagus, crop (that stores the food), and the gizzard (that grinds the food) are the parts of this region. The ingestion and the mastication of the food is the prime function of the foregut. The paired salivary glands are situated on each side of the oesophagus. These secrete saliva rich in enzymes. One pair of appendages called the mandibles function as jaws.
2. Midgut or Mesenteron: The absorption and digestion of the food occur in this region. At the junction of the gizzard and the midgut, the tubular six to eight structures arise that are called hepatic caecae. These caecae secrete the digestive enzymes into the midgut.
3. Hindgut or Proctodaeum: It consists of ileum, colon, rectum and the anal opening. The undigested food from the midgut is passed on to this region and is ejected out of the body via anus.

EXCRETORY SYSTEM

There are the following types of excretory organs present among different Arthropods:

1. Coxal Glands: These are found in the arachnids (Fig. 1a).
2. Green Glands: These are also called the maxillary glands and are present in a variety of crustaceans. There are three parts seen in this: labyrinth, end sac and bladder (Fig. 1b)
3. Malpighian Tubules: These are the main excretory organs in the insects. These are yellow-coloured tubules that arise from the junction of the midgut and the hindgut. These perform the uricotelic excretion (Fig. 1c)

RESPIRATORY SYSTEM

There are the following respiratory organs in Arthropods:

1. Book-gills: These are present in aquatic Arthropods. The crescent-shaped gills are arranged like a stack of book and enclose a gill chamber where the exchange of gases takes place.
2. Book-lungs: These are present in terrestrial forms. Four pairs of book-lungs open to the outer space via stigmata.
3. Trachea: These are the tubular structures seen in the insects. These open to the outer space via spiracles that allow diffusion of gases. The trachea branches to form the tracheoles in the deeper organs.

CIRCULATORY SYSTEM

It is of open type, i.e. the blood does not flow in closed vessels. The blood fills the body cavity to form the haemocoel. The dorsal blood vessel represents the heart. It is neurogenic.

SENSE ORGANS

There are following two types of sense organs:

1. Sensillae: These are seen in insects and are present in the skin. They respond to touch, taste and smell.
2. Eyes: The compound eyes made up of many ommatidia are seen in the Arthropods. Each ommatidium forms its own vision, so the overall vision is of mosaic type.

FERTILIZATION AND DEVELOPMENT

The Arthropods are unisexual animals. The sperms are delivered into the body of the female in groups called spermatophores. The fertilization is generally internal. The fertilized eggs develop with the formation of the larval stage, so the development is indirect.

CLASSIFICATION

On the basis of divisions of the body and the types of appendages, there are seven classes under this phylum:

 ECONOMICALLY IMPORTANT INSECTS

1. Butterflies, ants, wasps, bees etc. act as pollinating agents.
2. Honey, was, propolis and royal jellies are obtained from the honey bees.
3. Silk is obtained from the cocoon of the larvae of the silk moth.
4. Lac is obtained from the lac insect.                                              

PHYLUM MOLLUSCA

GENERAL FEATURES

• This phylum contains the second largest group of animals.
• They occur in oceans, fresh water and on land.
• The body is unsegmented. The following three features are distinctly visible
  • Head
  • Visceral hump
  • Muscular foot
• A fleshy and folded structure called the mantle covers the body.
• Their body is externally covered with the shell that is secreted by the mantle and is made up of calcium carbonate.
• The coelom is greatly reduced and is replaced by the haemocoel.

DIGESTIVE SYSTEM

They have a complete digestive system with a distinct mouth and anus. In some animals, a special rasping organ called the radula is present. It has chitinous teeth that help in scraping.

CIRCULATORY SYSTEM

They have an open type of blood vascular system (except cuttlefish that have closed circulatory system). The presence of a respiratory pigment called haemocyanin makes the blood blue. The heart is dorsally located. The arteries open into the sinuses.

RESPIRATORY SYSTEM

The following respiratory organs can be seen in the animals:

1. Skin: In the parasitic forms like Cenia, the skin acts as the respiratory system.
2. Ctenidium: These are present in the aquatic animals in the mantle cavity. These are comb-like structures.
3. Pulmonary sac: These are present in the terrestrial forms.

NERVOUS SYSTEM AND SENSE ORGANS

The following types of ganglia make up their nervous system:

1. Cerebral ganglia
2. Pleural ganglia
3. Pedal ganglia
4. Visceral ganglia

The dissimilar ganglia are joined by the connectives while the commissures join the similar ganglia. The eyes located on the stalk called ommatophores are main sense organs. Statocysts for balancing and osphradium for chemical stimulus are also seen as sense organs.

REPRODUCTION AND DEVELOPMENT

Generally, the animals are unisexual. Some cephalopods and gastropods are hermaphroditic. The fertilization can be internal or external. The development of the embryo is indirect and larva formation occurs. The trocophore is the first larval stage and veliger is the second larval stage.

CLASSIFICATION

LIVING FOSSIL NEOPILINA

It has metameric or the segmented body which is not found in other Molluscs. It was discovered from the western coast of Mexico. It is the feature related to the Annelida. It has eight pairs of muscles and five to six pairs of gills. The five pairs of nephridia are present, similar to the Annelida.  These features make them the oldest Mollusca and are called the living fossils. They also establish Annelida as the ancestors to the Mollusca.

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