Difference Between Mollusks and Arthropods

Mollusks vs Arthropods

Identifying the difference between mollusks and arthropods is not so difficult if you pay careful attention to the anatomy of each. Phylum Mollusca and Phylum Arthropoda are two major vertebrate phyla that include the highest number of species’ diversity than any phyla in Animal Kingdom. Due to this huge diversity, people find difficult to classify them under the correct phyla. This article analyses the anatomy of each and identify the structural features that help to differentiate between mollusks and arthropods.

What are Mollusks?

Phylum Mollusca is considered as one of the largest phylum and is second only to Phylum Arthropoda. The phylum 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 and bilateral symmetry. The most common examples for mollusks include snails, clams and squids. 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. Gas exchange is taken place in gills. Mollusks have an open circulatory system in which the heart pumps blood into the open space around the body organs. 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 that are used to catch prey and for movement. Three major classes of Phylum Mollusca includes 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 are adapted to different activities including feeding, capturing prey, mating, and sensory actions depending on the environment they live. These creatures are cosmopolitan and their body size vary from microscopic mites to large Japanese spider crabs. Arthropods exhibit bilateral symmetry, segmented body, body cavity, a nervous system, a digestive system and an exoskeleton. The hard exoskeleton is made up of chitin and provides protection, support, and covering for internal body organs and also provides muscle attachment sites. Since, exoskeleton restrict their growth, arthropods molt their exoskeleton regularly. The most recent classification of Phylum Arthropoda has four groups, namely; Chelicerata (spiders, mites, and scorpions), Crustacea (shrimps, crabs, lobsters, and water fleas), Hexapoda (insects, springtails and relatives) and Myriapoda (millipedes and centipedes).

Difference Between Mollusks and Arthropods

Honey Bee

What is the difference between Mollusks and Arthropods?

• Phylum Arthropoda is the largest phylum of Animal Kingdom, whereas Phylum Mollusca is the second largest.

• Mollusks are the soft-bodied invertebrates with one or two shells, whereas arthropods are the invertebrates with jointed legs and appendages.

• Mollusks include snails, clams and squids. Arthropods include spiders, mites, scorpions, shrimps, crabs, lobsters, insects, etc.

• Unlike the arthropods, some mollusk species have muscular foot for locomotion.

• Unlike mollusks, arthropods like insects have wings to fly.

• Mollusks have mantles that secrete calcareous outer or inner shell. Arthropods have exoskeleton made up chitin.


Images Courtesy: 

  1. Squid by Nhobgood (CC BY-SA 3.0)
  2. Honey bee by Parent Géry (CC BY-SA 4.0)