🕷️ Phylum Arthropoda (Jointed-legged Animals)
Arthropoda is the largest phylum in the animal kingdom, comprising nearly 80% of all known species. Members are found in every habitat — terrestrial, aquatic, and aerial.
- Organ Systems:
- Circulatory System: Is of open type. Blood (hemolymph) flows through the body cavity (haemocoel).
- Respiratory Organs: Respiration occurs through various structures: gills (aquatic forms like prawns), book gills (e.g., Limulus - King crab), book lungs (e.g., spiders, scorpions), or a network of tracheal tubes (insects).
- Excretory Organs: Excretion takes place through Malpighian tubules (in insects and myriapods), green glands or antennal glands (in crustaceans), or coxal glands (in arachnids).
- Sensory Organs: Include antennae (touch, smell), eyes (simple and compound), statocysts (balance organs), and auditory organs.
- Nervous System: Similar to annelids, with a nerve ring and a double ventral nerve cord with ganglia.
🦋 General Characteristics
- Level of Organisation: They exhibit the organ-system level of organisation.
- Symmetry: They are bilaterally symmetrical.
- Germ Layers: They are triploblastic.
- Segmentation: They are segmented animals.
- Coelom: They are coelomates.
- Exoskeleton: The body is covered by a hard, protective chitinous exoskeleton. This provides support and protection but requires shedding (moulting or ecdysis) for growth.
- Body Division: The body typically consists of head, thorax, and abdomen, though in some (like crustaceans) the head and thorax are fused to form a cephalothorax.
- Jointed Appendages: They possess jointed appendages (arthros-joint, poda-appendages), which are modified for various functions like walking, feeding, sensory reception, defense, etc. This is the feature from which the phylum derives its name.
🌊 Reproduction
- Mostly dioecious (sexes are separate).
- Fertilisation is usually internal.
- Mostly oviparous (egg-laying), though some are viviparous.
- Development may be direct or indirect (involving larval stages, e.g., caterpillars in butterflies, grubs in beetles, nauplius in crustaceans).
🧠 Economic Importance
- Beneficial: Apis (Honey bee - produces honey and wax), Bombyx (Silkworm - produces silk), Laccifer (Lac insect - produces lac).
- Vectors: Many insects act as vectors transmitting diseases (e.g., Anopheles, Culex, Aedes - mosquitoes).
- Pests: Locusta (Locust - gregarious pest), various agricultural pests.
- Living Fossil: Limulus (King crab).
🧩 Examples
| Class | Common Name | Example |
|---|---|---|
| Crustacea | Aquatic forms | Prawn, Crab |
| Insecta | Terrestrial, 3 pairs of legs | Cockroach, Butterfly, Honeybee |
| Arachnida | 4 pairs of legs | Spider, Scorpion |
| Myriapoda | Many segments | Centipede, Millipede |
🐚 Phylum Mollusca (Soft-bodied Animals)
Molluscs are the second largest animal phylum, characterized by soft, unsegmented bodies often protected by an external calcareous shell.
Body Structure:
The body is covered by a calcareous shell (external or sometimes internal) and is unsegmented.
There is a distinct head, muscular foot, and visceral hump.
Mantle: A soft and spongy layer of skin forms a mantle over the visceral hump.
Mantle Cavity: The space between the hump and the mantle is called the mantle cavity. Feather-like gills (ctenidia) are present in the mantle cavity, serving both respiratory and excretory functions.
Head Region: The anterior head region has sensory tentacles.
Radula: The mouth contains a file-like rasping organ for feeding, called the radula.
Organ Systems:
Digestive System: Complete.
Circulatory System: Open type (except in Cephalopods like squid and octopus which have a closed system). Blood often contains a copper-containing pigment, haemocyanin.
Respiratory Organs: Usually feather-like gills (ctenidia) in the mantle cavity. Terrestrial forms may have lungs.
Excretory Organs: Typically metanephridia, often called organs of Bojanus or Keber's organ.
Nervous System: Consists of paired ganglia connected by nerves.
Sensory Organs: Tentacles, eyes (simple or complex, e.g., in Octopus), and statocysts (balance).
🦪 General Characteristics
- Habitat: Molluscs are terrestrial or aquatic (marine or fresh water).
- Level of Organisation: They exhibit the organ-system level of organisation.
- Symmetry: They are bilaterally symmetrical.
- Germ Layers: They are triploblastic.
- Coelom: They are coelomate animals.
🌊 Reproduction
- Usually dioecious (sexes separate) and oviparous (egg-laying).
- Fertilisation can be external or internal.
- Development is often indirect through larval stages (e.g., trochophore, veliger larva).
🧩 Examples
Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea-hare), Dentalium (Tusk shell), Chaetopleura (Chiton).
| Class | Example | Distinguishing Feature |
|---|---|---|
| Pelecypoda | Unio | Bivalve shell (clams, mussels) |
| Gastropoda | Pila (snail) | Asymmetrical body, coiled shell |
| Cephalopoda | Octopus, Sepia | Highly developed head, closed circulation |
Economic Importance:
- Food: Unio, Sepia
- Pearls: Pinctada
- Ornamentation and lime industry
🔬 Comparison: Arthropoda vs Mollusca
| Feature | Arthropoda | Mollusca |
|---|---|---|
| Body covering | Chitinous exoskeleton | Calcareous shell |
| Segmentation | Present | Absent |
| Circulation | Open | Open/Closed |
| Coelom | True coelomate | True coelomate |
| Locomotion | Jointed appendages | Muscular foot |
| Example | Cockroach, Prawn | Pila, Octopus |
💡 Questions and Answers
Q1. What are Malpighian tubules?
A1 Malpighian tubules are the primary excretory organs found in insects and other terrestrial arthropods like myriapods. They are thin tubules, closed at the distal end and opening into the junction of the midgut and hindgut. They absorb waste products (like uric acid) and ions from the surrounding hemolymph and pass them into the hindgut for elimination along with faeces, helping in both excretion and osmoregulation.
Q2. Differentiate between open and closed circulatory systems.
A2.
- Open Circulatory System: Found in Arthropods and most Molluscs. Blood (hemolymph) is pumped by the heart into open spaces (sinuses or hemocoel) that directly bathe the tissues. Blood returns to the heart through openings (ostia).
- Closed Circulatory System: Found in Annelids and Vertebrates. Blood remains confined within a network of vessels (arteries, veins, capillaries) throughout its circulation.
Q3. Name four different types of respiratory organs found in Arthropods and give an example for each.
A3.
- Gills: Found in aquatic arthropods. Example: Prawns (Palaemon).
- Book Gills: Leaf-like gills arranged like pages in a book. Example: King crab (Limulus).
- Book Lungs: Internal, leaf-like structures for aerial respiration. Example: Spiders, Scorpions.
- Tracheal System: A network of air tubes (tracheae) that branch throughout the body, delivering oxygen directly to tissues. Example: Insects (Cockroach, Grasshopper).
Q4. What is moulting (ecdysis) and why is it necessary for arthropods?
A4. Moulting or ecdysis is the process of shedding the old, hard chitinous exoskeleton. It is necessary because the exoskeleton is rigid and non-living, and does not grow with the animal. To increase in size, the arthropod must periodically shed its old exoskeleton and secrete a new, larger one, during which time it is soft and vulnerable.
Q5. What is the mantle and mantle cavity in Molluscs? What are their functions?
A5.
- The mantle is a soft, fleshy layer of skin covering the visceral hump. It typically secretes the calcareous shell.
- The mantle cavity is the space between the mantle and the visceral hump.
- Functions: The mantle cavity usually houses the feather-like gills (ctenidia) which function in respiration and also aid in excretion. It is also the site where digestive, excretory, and reproductive systems discharge their products.
Q6. What is a radula? What is its function?
A6. The radula is a unique feeding organ found in the mouth of most molluscs (except bivalves). It is a minutely toothed, chitinous ribbon, typically used to scrape or rasp food particles off surfaces and draw them into the mouth.
Q7. Describe the circulatory system typical of Molluscs.
A7. Most molluscs have an open circulatory system. The heart pumps blood (often called hemolymph) through some vessels, but it then enters open spaces or sinuses within the body tissues (hemocoel) before returning to the heart. An exception is the Cephalopod class (squids, octopuses), which have evolved a closed circulatory system.
Q8. List the three distinct body parts of a typical mollusc.
A8. The unsegmented body of a typical mollusc is divisible into three distinct regions:
- Head: Contains sensory organs (like tentacles, eyes) and the mouth.
- Visceral Hump (Visceral Mass): Contains the main body organs (digestive, reproductive, excretory).
- Muscular Foot: A large, ventral structure primarily used for locomotion, burrowing, or capturing prey.