🦀 Arthropods: Crustaceans & Arachnids 🕷️

A Deep Dive into Two Remarkable Classes of the Phylum Arthropoda

🔄 Quick Recap from Last Lesson

You already know that arthropods are the largest phylum in the animal kingdom. They all share: a chitinous exoskeleton, segmented bodies, jointed appendages, and an open circulatory system. They also molt (shed their exoskeleton) to grow. Today, we zoom in on two fascinating classes: Crustaceans and Arachnids.

🦞 Class Crustacea — The Crustaceans

Crustacea includes familiar organisms such as crabs, lobsters, shrimp, crayfish, and barnacles. Let's explore their anatomy and biology in detail.

📐 Body Plan & Segmentation

The body of crustaceans is usually divided into three main regions:

Head (Cephalon): Contains the brain, eyes (often compound and on stalks), and two pairs of antennae located in front of the mouth — this is a defining feature unique to crustaceans among all arthropods.
Thorax: Bears the walking legs and feeding appendages. In many species, the head and thorax are fused into a cephalothorax covered by a hard shield called the carapace.
Abdomen: Often used for swimming; ends in a tail fan (telson) in many species like lobsters and shrimp.

🔑 Key Fact: All appendages in crustaceans are biramous, meaning they are divided into two branches — an inner branch (endopod) and an outer branch (exopod). This is different from insects, whose limbs are unbranched (uniramous).

Figure 1: External anatomy of a typical decapod crustacean (e.g., lobster)

🫁 Respiration & Internal Systems

Breathing: The respiration of crustaceans occurs through gills, which are feathery structures usually located at the base of the legs, protected under the carapace.
Circulatory System: Like all arthropods, crustaceans have an open circulatory system — blood (called hemolymph) is pumped by the heart into open body cavities rather than through closed vessels.
Nervous System: A ventral nerve cord with ganglia (nerve clusters) in each segment, plus a brain in the head.
Digestion: Complete digestive tract with a specialized stomach that grinds food using a structure called the gastric mill.

🌍 Habitat & Diversity

Crustaceans are found in marine, freshwater, and terrestrial environments, though the vast majority are aquatic.

🏆 Record Holders: The largest crustaceans belong to the order Decapoda (~10,000 species). The American lobster can reach 20 kg (44 lbs), and the giant Japanese spider crab has legs spanning up to 3.7 metres!

🐛 Surprise! The woodlouse (pill bug / roly-poly) is actually a crustacean that lives on land! It has a highly segmented, armored body but cannot survive arid conditions — that's why you find them in damp crevices. They breathe through gill-like structures even on land.

🔬 Unique Larval Stage: A defining feature of crustaceans is a unique larval stage common to all members of this subphylum. Most crustaceans hatch as a tiny nauplius larva — a free-swimming form with only three pairs of appendages. This larva molts many times before reaching adult form, a process called metamorphosis.

Arthropods: crustaceans and arachnids