Invertebrate Phyla: Arthropoda, Mollusca, and Hemichordata
Phylum Arthropoda: Jointed Appendages and Exoskeletons
Phylum Arthropoda is the largest phylum in the animal kingdom, comprising more than 80% of all known species. Their hallmark is the presence of jointed appendages and a chitinous exoskeleton.
General Characters of Phylum Arthropoda
- Level of Organization: Organ-system level.
- Symmetry and Germ Layers: Bilaterally symmetrical, triploblastic, and metamerically segmented.
- Exoskeleton: A hard outer covering made of chitin, which provides protection and is periodically shed through a process called ecdysis or moulting.
- Body Cavity: The true coelom is reduced; the body cavity is a haemocoel filled with blood (haemolymph).
- Circulatory System: Open type, with a dorsal heart.
- Respiration: Through gills (crustaceans), tracheae (insects), or book lungs/book gills (arachnids).
- Excretion: Via Malpighian tubules or green glands.
- Nervous System: Consists of a nerve ring and a solid double ventral nerve cord with segmental ganglia.
Classification (Up to Class Level)
Arthropods are typically divided into several classes based on their body regions (tagmata) and appendages.
| Class | Body Regions | Appendages | Examples |
|---|---|---|---|
| Crustacea | Cephalothorax & Abdomen | Two pairs of antennae; many pairs of legs. | Prawn, Crab |
| Insecta | Head, Thorax & Abdomen | One pair of antennae; three pairs of legs. | Cockroach, Ant |
| Arachnida | Cephalothorax & Abdomen | No antennae; four pairs of legs. | Spider, Scorpion |
| Chilopoda | Head & Trunk | One pair of legs per segment. | Centipede |
| Diplopoda | Head & Trunk | Two pairs of legs per segment. | Millipede |
Type Study: Cockroach (Periplaneta americana)
Habitat and Habits
- Habitat: Nocturnal, omnivorous insects found in warm, damp, and dark places like kitchens, sewers, and warehouses.
- Habits: Fast runners (cursorial) and capable of flight (though they rarely do so). They are dioecious (separate sexes).
External Morphology
- Body: Flattened, reddish-brown, and divided into Head, Thorax, and Abdomen.
- Head: Small, triangular, and bears a pair of long antennae, compound eyes, and biting/chewing mouthparts.
- Thorax: Divided into Prothorax, Mesothorax, and Metathorax. Each bears a pair of walking legs. Two pairs of wings (tegmina and hindwings) arise from the meso- and metathorax.
- Abdomen: Consists of 10 segments in both sexes. Both have a pair of anal cerci; only males have anal styles (used for sexual dimorphism).
Digestive System
The alimentary canal is divided into:
- Foregut: Mouth → Pharynx → Oesophagus → Crop (for storage) → Gizzard (contains 6 chitinous teeth for grinding).
- Midgut (Mesenteron): A narrow tube where digestion and absorption occur. At the junction of the foregut and midgut are 6–8 Hepatic caecae which secrete digestive enzymes.
- Hindgut: Divided into Ileum, Colon, and Rectum. It opens out through the anus.
Respiratory System
- Tracheal System: A network of air tubes (tracheae) that branch into tracheoles to deliver oxygen directly to tissues.
- Spiracles: 10 pairs of small lateral openings (2 thoracic, 8 abdominal) that regulate air intake.
Excretory System
- Malpighian Tubules: 100–150 yellow filamentous tubules at the junction of the midgut and hindgut. They extract nitrogenous waste from the haemolymph and convert it into uric acid (making the cockroach uricotelic).
Reproductive System
- Male: Pair of testes (segments 4–6), a mushroom-shaped gland, and a pair of seminal vesicles. Sperm is stored in spermatophores.
- Female: Pair of ovaries (segments 2–6), each containing 8 ovarioles. Eggs are fertilized and encased in a dark-colored capsule called an ootheca.
Phylum Mollusca: Soft-Bodied Invertebrates
Phylum Mollusca (from Latin mollis, meaning “soft”) is the second-largest phylum of the animal kingdom. It includes familiar creatures like snails, clams, squids, and octopuses.
General Characters
- Body Form: Soft-bodied, unsegmented, and typically bilaterally symmetrical.
- Triploblastic and Coelomate: They have three germ layers and a true coelom, though the coelom is often reduced to small spaces around the heart and gonads (haemocoel).
- Body Divisions: The body is divided into four main regions:
- Head: Contains sense organs (eyes, tentacles) and the mouth.
- Muscular Foot: Used for locomotion (crawling, burrowing, or swimming).
- Visceral Mass: Contains the internal organs.
- Mantle (Pallium): A thin fleshy fold that covers the visceral mass and secretes the calcareous shell.
- Radula: A unique, rasping organ in the mouth used for feeding (absent in bivalves).
- Respiration: Occurs through gills (ctenidia) in aquatic forms or a pulmonary sac (lung) in terrestrial forms.
- Circulatory System: Usually open type (except in cephalopods), with a heart and blood sinuses.
- Excretion: Performed by one or two pairs of kidneys (Metanephridia).
Classification (Up to Class Level)
Mollusks are classified primarily based on the characteristics of the foot, shell, and mantle.
| Class | Shell Type | Locomotory Foot | Examples |
|---|---|---|---|
| Monoplacophora | Single dome-shaped | Broad and flat | Neopilina |
| Polyplacophora | 8 overlapping plates | Large, flat, ventral | Chiton |
| Aplacophora | Absent (spicules only) | Reduced or absent | Chaetoderma |
| Gastropoda | Single, usually spirally coiled | Large, flat, used for creeping | Pila (Apple Snail), Helix |
| Bivalvia | Two hinged valves | Wedge-shaped (for digging) | Unio, Pinctada (Pearl Oyster) |
| Scaphopoda | Tubular (Tusk-like) | Conical | Dentalium (Tusk shell) |
| Cephalopoda | Internal, reduced, or absent | Modified into tentacles | Octopus, Sepia (Cuttlefish) |
Type Study: Pila globosa (Apple Snail)
Pila is a common freshwater snail found in ponds and rice fields. It is known for its amphibious nature.
External Morphology
- The Shell: A large, globose, spirally coiled shell. It shows dextral (right-handed) coiling. The opening is closed by a calcareous lid called the operculum.
- Body: Divisible into head, foot, and visceral mass. The head bears two pairs of tentacles and a pair of eyes on stalks (ommatophores).
- Nuchal Lobes: Two fleshy projections (left and right) used as siphons during respiration.
Digestive System
- Consists of a tubular alimentary canal and digestive glands (salivary glands and a large hepatopancreas).
- The buccal mass contains the radula, which has transverse rows of chitinous teeth for rasping aquatic vegetation.
Respiratory System (Double Respiration)
Pila is unique because it can breathe in both water and air:
- Aquatic Respiration: Uses a single comb-like gill (ctenidium) located in the mantle cavity.
- Aerial Respiration: Uses a pulmonary sac (lung). When in water with low oxygen or on land, it uses its left nuchal lobe to form a long tube (siphon) to draw in atmospheric air.
Nervous System
- Shows a “figure-of-8” arrangement due to torsion (the 180° twisting of the visceral mass during development).
- Consists of paired ganglia: Cerebral, Buccal, Pedal, Pleural, and Visceral.
Reproductive System
- Dioecious: Sexes are separate, but sexual dimorphism is not very distinct.
- Males have a penis for internal fertilization; females lay eggs in moist soil near water.
Phylum Hemichordata: The Worm-Like Bridge Phylum
Phylum Hemichordata (meaning “half-chordates”) is a small group of worm-like marine animals. For a long time, they were classified as a subphylum under Chordata because scientists believed the “stomochord” in their head was a primitive notochord. However, it is now recognized as a separate phylum that bridges the gap between Echinoderms and Chordates.
General Characters of Hemichordates
- Habitat: Exclusively marine. Most species live in U-shaped burrows in sandy or muddy bottoms (tubicolous), while others are colonial and sedentary.
- Body Division: The body is soft, cylindrical, and divided into three distinct regions:
- Proboscis: The anterior-most, club-shaped part used for burrowing and feeding.
- Collar: A short, muscular middle region.
- Trunk: The long posterior part containing most of the internal organs (visceral organs).
- Symmetry and Organization: They are bilaterally symmetrical, triploblastic, and possess an organ-system level of organization.
- Stomochord: They possess a hollow outgrowth of the gut that extends into the proboscis, known as the stomochord (previously mistaken for a notochord).
- Respiration: Carried out through several pairs of pharyngeal gill slits.
- Circulatory System: An open type consisting of a dorsal heart and two longitudinal vessels (one dorsal and one ventral).
- Excretion: Performed by a single glomerulus (also called the proboscis gland) located in the proboscis.
- Nervous System: Primitive, consisting of a sub-epidermal nerve plexus. A dorsal “nerve cord” exists in the collar region.
- Reproduction: Mostly dioecious (separate sexes). Fertilization is usually external in seawater.
- Development: Often includes a free-swimming ciliated larva called the Tornaria larva, which closely resembles the bipinnaria larva of echinoderms.
Classification and Examples
The phylum is primarily divided into two main classes:
Class 1: Enteropneusta (Acorn Worms)
- Features: Solitary, burrowing worms with a long trunk and numerous gill slits. They have a straight alimentary canal.
- Examples: Balanoglossus (Commonly called the “Tongue worm” or “Acorn worm”), Saccoglossus.
Class 2: Pterobranchia
- Features: Small, often colonial animals that live in tubes (zooids). They have a U-shaped alimentary canal and the collar bears ciliated arms (tentacles) for filter feeding.
- Examples: Rhabdopleura (Colonial), Cephalodiscus (Solitary or social).
Evolutionary Significance
Hemichordates are considered a phylogenetic link between invertebrates (specifically Echinoderms) and Chordates. They share:
- With Echinoderms: Similarities in larval stages (Tornaria) and early embryonic development.
- With Chordates: Pharyngeal gill slits and a dorsal hollow nerve cord (though only in the collar region).