Difference Between Arachnids and Crustaceans

The main difference between arachnids and crustaceans is that arachnids are mainly terrestrial animals whereas crustaceans are mainly aquatic. However, both arachnids and crustaceans are two groups of invertebrates with similar body structures; an exoskeleton, a segmented body, and jointed appendages. Therefore, differentiating arachnids from crustaceans is a bit tricky. But, arachnids body has two sections: opisthosoma and prosoma while Crustacean’s body has three sections: the head, thorax, and abdomen.

Key Areas Covered

1. What are Arachnids
     – Definition, Characteristics
2. What are Crustaceans
     – Definition, Characteristics
3. What are the Similarities Between Arachnids and Crustaceans
     – Outline of Common Features
4. What is the Difference Between Arachnids and Crustaceans
    – Comparison of Key Differences

Key Terms: Appendages, Arachnids, Arthropods, Body Segments, Exoskeleton, Habitat

What are Arachnids 

Arachnids are wingless, terrestrial arthropods with two body segments. The two segments are called opisthosoma (abdomen) and prosoma (cephalothorax). Over 60,000 arachnid species are found on earth. Arachnids have eight jointed legs for walking. The prosoma consists of a pair of pre-oral chelicerae and a pair of post-oral pedipalps. The other two pairs of legs are found in the abdomen.

Figure 1: Spider

The respiration of arachnids occurs through book lungs while excretion occurs through coxal glands or malpighian tubules. Some common arachnids are acari, spider, scorpion, tics, and mites.

What are Crustaceans

Crustaceans are mainly marine aquatic arthropods whose body is segmented into head, cephalothorax, and thorax. Over 30,000 species of crustaceans are found on earth. The cephalothorax is covered with a large carapace. One of the characteristic features of crustaceans among other arthropods is the presence of two antennae. Five pairs of appendages are connected to the cephalothorax. The respiration of crustaceans occurs through gills. Excretion occurs through antennal glands or green glands. Crayfish, crabs, shrimp are examples of crustaceans.

Figure 2: Crayfish

Similarities Between Arachnids and Crustaceans

• Arachnids and crustaceans are arthropods.
• Both are invertebrates.
• The two invertebrates have a segmented body covered by an exoskeleton.
• Both have bilateral symmetry.
• Arachnids and crustaceans are triploblastic, haemocoelomic
• Both arachnids and crustaceans have a complete digestive system.
• They are both cold-blooded animals.

Difference Between Arachnids and Crustaceans

Definition

Arachnids: Arachnids are wingless, terrestrial arthropods with two body segments.

Crustaceans: Crustaceans are mainly marine aquatic arthropods whose body is segmented into head, cephalothorax, and thorax.

Number of Species

Arachnids: Over 60,000 arachnid species are found on earth.

Crustaceans: Over 30,000 crustacean species are found on earth.

Habitat

Arachnids: Arachnids are mainly terrestrial.

Crustaceans: Crustaceans are mainly marine.

Segmentation

Arachnids: Arachnids have two body segments: opisthosoma and prosoma.

Crustaceans: Crustaceans have three body segments: head, cephalothorax, and abdomen.

Size

Arachnids: Arachnids are less than 5 cm in size.

Crustaceans: Crustaceans are larger than arachnids.

Appendages 

Arachnids: Arachnids use their appendages to walk.

Crustaceans: Crustaceans use their appendages to swim, crawl, manipulate food, etc.

Feeding

Arachnids: Arachnids are nocturnal predators.

Crustaceans: Crustaceans are carnivores and herbivores.

Respiration

Arachnids: The respiration of arachnids occurs through book lungs.

Crustaceans: The respiration of crustaceans occurs through gills.

Excretion

Arachnids: The excretion of arachnids occurs through coxal glands.

Crustaceans: The excretion of crustaceans occurs through antennal glands or green glands.

Conclusion

Arachnids and crustaceans are two groups of arthropods that are invertebrates. Arachnids are terrestrial animals whereas crustaceans are aquatic animals. Both arachnids and crustaceans have a segmented body with jointed appendages. The main difference between arachnids and crustaceans is the type of habitat they live.

Reference:

1. “Arachnids.” Target Study, Available here.
2. “Crustacea (Crustaceans) – General Characters (Phylum Arthropoda).” Easybiologyclass, Available here.

Image Courtesy:

1. “87237” (CC0) via Pixabay
2. “Crayfish2” by U.S. Department of Agriculture (CC BY 2.0) via Flickr