Unit 4: Arthropoda
1. Phylum Arthropoda
Arthropoda (from Greek: arthron = joint, podos = foot) is the largest phylum in the animal kingdom, including insects, arachnids, crustaceans, and others. Their success is attributed to a number of key evolutionary innovations.
1.1 General Characters and Classification
General Characters
- Exoskeleton: A hard, protective outer covering made of chitin. It provides support, prevents water loss, and serves as an attachment point for muscles. It must be periodically shed (molting or ecdysis) to allow for growth.
- Jointed Appendages: Paired, jointed limbs that are modified for various functions like walking, feeding, swimming, and sensing.
- Segmentation and Tagmosis: The body is metamerically segmented, but segments are often fused into specialized body regions called tagmata (e.g., head, thorax, and abdomen in insects).
- Circulatory System: Open type. Blood (hemolymph) flows through a dorsal heart into open sinuses (the hemocoel) that bathe the tissues directly.
- Respiratory Organs: Diverse, including gills (aquatic), book lungs (arachnids), book gills (Limulus), and tracheae (insects, myriapods).
- Nervous System: Annelid-like, with a dorsal brain and a double ventral nerve cord.
- Sense Organs: Well-developed, including antennae, simple eyes (ocelli), and compound eyes.
Classification of Arthropoda (Up to Classes)
Classification is based on body division, type of appendages, and respiratory organs.
| Subphylum/Class |
Key Characteristics |
Examples |
| Class Crustacea |
Mostly aquatic; two pairs of antennae; respiration by gills; exoskeleton hardened with calcium carbonate. Tagmata: Cephalothorax and abdomen. |
Prawn (Palaemon), Crab (Cancer), Daphnia |
| Class Myriapoda |
Terrestrial; numerous segments, each with legs. Tagmata: Head and trunk. One pair of antennae. Respiration by tracheae. |
Centipedes (Scolopendra), Millipedes (Julus) |
| Class Insecta (Hexapoda) |
Largest class. Terrestrial; one pair of antennae; three pairs of legs. Tagmata: Head, thorax, abdomen. Respiration by tracheae. |
Cockroach, Butterfly, Mosquito, Grasshopper |
| Class Arachnida |
Terrestrial; no antennae; four pairs of legs. Tagmata: Cephalothorax and abdomen. Respiration by book lungs or tracheae. |
Spider, Scorpion, Tick, Mite |
1.2 Digestion, Vision and Respiration in Arthropoda (with special reference to Prawn)
Digestion in Prawn
The prawn's digestive system is a straight tube divided into foregut, midgut, and hindgut.
- Foregut (Stomodeum): Consists of a mouth, oesophagus, and a large stomach. The stomach is divided into a cardiac stomach and a pyloric stomach. The cardiac stomach contains a chitinous gastric mill apparatus that grinds food.
- Midgut (Mesenteron): The main site of digestion and absorption. A large digestive gland, the hepatopancreas, secretes enzymes into the midgut.
- Hindgut (Proctodeum): A short tube that absorbs water and forms fecal pellets, which are egested through the anus.
Vision in Prawn
Prawns possess a pair of large, stalked compound eyes.
- Structure: Each compound eye is made of numerous independent visual units called ommatidia. Each ommatidium has a lens, a crystalline cone, and a group of light-sensitive retinular cells.
- Image Formation: They form a mosaic image, where each ommatidium contributes a small piece of the overall picture. This type of vision is excellent for detecting movement but provides lower resolution than a simple lens eye.
- Types of Vision:
- Apposition Image (Bright light): Each ommatidium works independently, creating a sharp but dim point-by-point image.
- Superposition Image (Dim light): Light from several ommatidia is focused onto a single rhabdome, creating a brighter but less distinct, overlapping image.
Respiration in Prawn
As an aquatic arthropod, the prawn respires using gills (branchiae).
- Location: The gills are located in gill chambers on either side of the cephalothorax, protected by the carapace.
- Mechanism: A constant current of water is maintained over the gills by the beating of an appendage called the scaphognathite (or gill bailer).
- Gas Exchange: The gills are richly supplied with blood (hemolymph). Deoxygenated hemolymph from the body flows into the gills, where it releases CO₂ and picks up dissolved oxygen from the water via diffusion. The oxygenated hemolymph then returns to the heart to be circulated throughout the body.
2. Connecting Links
Connecting links are organisms that possess characteristics of two different taxonomic groups, providing evidence for evolutionary relationships.
2.1 Characteristics and Phylogenetic Significance of Onychophora (e.g., Peripatus)
Onychophorans, or velvet worms, are a small phylum of terrestrial invertebrates that are a classic example of a connecting link between Annelida and Arthropoda.
Annelidan Characteristics (Primitive)
- Worm-like, unjointed body.
- Thin, flexible cuticle (not heavily sclerotized like arthropods).
- Segmentally arranged nephridia for excretion.
- Presence of cilia in the reproductive tract.
- Simple, unjointed, lobe-like legs (lobopods).
Arthropodan Characteristics (Advanced)
- Presence of antennae.
- Growth by molting (ecdysis) of the cuticle.
- Jaw-like mandibles derived from appendages.
- Presence of a tracheal system for respiration.
- Reduced coelom and a large hemocoel (open circulatory system).
Phylogenetic Significance
Peripatus strongly suggests that arthropods evolved from an annelid-like ancestor. It represents an intermediate stage, demonstrating how a segmented, worm-like body plan could give rise to the more complex, sclerotized body plan of arthropods.
2.2 Characteristics and Phylogenetic Significance of Limulus (Horseshoe Crab)
Limulus, the horseshoe crab, is not a true crab (which is a crustacean) but belongs to the Subphylum Chelicerata, along with spiders and scorpions. It is considered a "living fossil" because it has changed very little over hundreds of millions of years.
Key Characteristics
- Large, horseshoe-shaped carapace covering the prosoma (cephalothorax).
- Abdomen (opisthosoma) is unsegmented and ends in a long tail spine (telson).
- Respiration through book gills located on the abdominal appendages.
- Has both compound eyes and simple eyes.
- The first pair of appendages are chelicerae (pincers for feeding).
Phylogenetic Significance
- Link to Extinct Groups: Limulus is closely related to the extinct trilobites and eurypterids (giant water scorpions), providing valuable insights into the biology of these ancient arthropods.
- Primitive Arthropod Features: It retains many primitive characteristics of the Chelicerata lineage, helping us understand the early evolution of this group.
- Medical Importance: The blue, copper-based blood of Limulus contains amebocytes. An extract from these cells, called Limulus Amebocyte Lysate (LAL), is used worldwide to detect bacterial endotoxins in medical equipment and injectable drugs.
Exam Tip: Differentiate clearly between connecting links and living fossils. Connecting links (like Peripatus) bridge two different groups. Living fossils (like Limulus) are modern species that show close resemblance to ancient, extinct ancestors.