Difference Between Mollusks and Arthropods

The key difference between mollusks and arthropods is that mollusks are soft-bodied invertebrates with one or two shells while arthropods are animals with segmented bodies, paired appendages and an exoskeleton.

Phylum Mollusca and Phylum Arthropoda are two major vertebrate phyla that include the highest number of species diversity than any phyla in the Animal Kingdom. Due to this huge diversity, people find difficult to classify them under the correct phyla. Thus, this article analyses the anatomy of each and identifies the structural features, to help identify the difference between mollusks and arthropods.

CONTENTS

1. Overview and Key Difference
2. What are Mollusks 
3. What are Arthropods
4. Similarities Between Mollusks and Arthropods
5. Side by Side Comparison – Mollusks vs Arthropods in Tabular Form
6. Summary

What are Mollusks?

Phylum Mollusca is one of the largest phyla in the Kingdom Animalia. It is second only to Phylum Arthropoda. Phylum Mollusca consists of more than 110,000 identified species, and they inhabit both terrestrial and aquatic environments on earth. Mollusks are soft-bodied invertebrates with one or two shells. Moreover, they show bilateral symmetry. The most common examples for mollusks include snails, clams and squids. Generally, all mollusks have a thin outer layer called the mantle, which surrounds the body organs located insides the visceral mass. Mantle secretes the protective shell of the body. Moreover, gas exchange takes place in gills.

Mollusks have an open circulatory system in which the heart pumps blood into the open space around the body organs. Besides, they have a prominent head with a mouth and sensory organs. Mollusks like snails have a well-developed muscular foot for movement and adhesion. Squids have tentacles to catch prey and for movement. Three major classes of Phylum Mollusca include Gastropoda (snails and conchs), Bivalvia (clams, oysters, and scallops), and Cephalopoda (squids, octopuses, cuttlefish, and chambered).

What are Arthropods?

Phylum Arthropoda is the largest group of animals with more than a million different species. The word arthropoda has the meaning of “jointed foot”. Apart from the jointed legs, arthropods have jointed appendages like antennae, claws, and pincers. These appendages help arthropods to carry out different activities, including feeding, capturing prey, mating, and sensory actions depending on the environment they live in. These creatures are cosmopolitan, and their body size varies from microscopic mites to large Japanese spider crabs.

Arthropods exhibit bilateral symmetry, a segmented body, a body cavity, a nervous system, a digestive system and an exoskeleton. Chitin is the main compound of the hard exoskeleton. The exoskeleton provides protection, support, and covering for the internal body organs and also provides muscle attachment sites. Since exoskeleton restricts their growth, arthropods molt their exoskeleton regularly. Phylum Arthropoda has four groups: Chelicerata (spiders, mites, and scorpions), Crustacea (shrimps, crabs, lobsters, and water fleas), Hexapoda (insects, springtails and relatives) and Myriapoda (millipedes and centipedes).

What are the Similarities Between Mollusks and Arthropods?

• Mollusks and arthropods belong to two phyla of Kingdom Animalia.
• Both groups contain invertebrates.
• Also, both show bilateral symmetry.
• Furthermore, both groups show great diversity.

What is the Difference Between Mollusks and Arthropods?

Mollusks are the soft-bodied invertebrates with one or two shells, whereas arthropods are the invertebrates with segmented bodies, jointed legs and appendages. So, this is the key difference between mollusks and arthropods. Phylum Arthropoda is the largest phylum of Animal Kingdom, whereas Phylum Mollusca is the second largest. Mollusks include snails, clams and squids while arthropods include spiders, mites, scorpions, shrimps, crabs, lobsters, insects, etc.

Moreover, their anatomical structure for locomation too adds to the difference between mollusks and arthropods. That is; unlike arthropods, some mollusk species have a muscular foot for locomotion. In contrast, arthropods like insects have wings to fly. Furthermore, mollusks have mantles that secrete calcareous outer or inner shell while arthropods have an exoskeleton made up chitin. So, this is also a difference between mollusks and arthropods.

Summary – Mollusks vs Arthropods

Phylum Molluska and Phylum Arthropoda are two major phyla of the Kingdom Animalia. Mollusks are soft-bodied animals with a shell while arthropods are animals with segmented bodies. They also have paired jointed appendages and an exoskeleton. Both groups include invertebrates that show bilateral symmetry. Thus, this summarizes the difference between mollusks and arthropods.

Reference:

1. Barnes, Robert D. “Arthropod.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 12 Feb. 2019, Available here.
2. “Mollusca.” Wikipedia, Wikimedia Foundation, 11 June 2019, Available here.

Image Courtesy:

1. “2983235” (CC0) via Pixabay
2. “These four arthropods, a flea, mite, tick, and a horsefly, are often the source of many vector-borne infectious diseases..” By CDC, Public Health Image Library via Public Domain Pictures.