An Overview of the Phyla of Animalia (Intro to Animals part 3)

Hey guys, this is part 3 of the Introduction to Animal Diversity posted by Neil and Sai earlier today. This post will cover the major phyla of Kingdom Animalia and give major features. It is assumed that you read and understand part 1 and 2 of this guide.

Last time we discussed Porifera, Cnidaria, Platyhelminthes, and Nematoda.

Now we will discuss Mollusca, Annelida, Arthropoda, and Echinodermata.


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Mollusca are named because they have a soft underside. This group includes snails, slugs, oysters, clams, Octopi, and Squid. Some members of this group have shells while others do not; shells are not a defining feature of this group. For those who do have shells, they are made of calcium carbonate secretions. Mollusca are coelomates and thus have a true digestive cavity. Mollusca are made of three main things:

  1. Muscular Foot
  2. Visceral Mass with organs inside
  3. Mantle tissue which drapes the animal and for some includes a shell.

Mollusca include groups such as Gastropods which move by rippling their foot or using cilia, Bivalves which have been divided into hinged animals with 2 shells, and Cephalopods (octopus and squid) which use their tentacles to hunt prey.


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Annelids are segmented worms. The name comes from their fused ringlike segments. They are coelomates because they have a completed digestive body cavity. There are a few main groups of Annelids.

  1. Errantians – mobile and aquatic organisms which parapodia paddle like structures for movement.
  2. Sedentaria – less mobile and tend to burrow in land or in marine soil

There are a few groups of Sedentaria such as Leeches and Earthworms.


Arthropods are one of the most successful groups of Animals and can be found throughout the entire world. This success is due to the structure of Arthropods which have hard exoskeletons, a segmented body, and connected appendages. The aforementioned appendages went through wide radial evolution to perform many different functions for the different global environments. Arthropods have an open circulatory system where Hemolymph blood is exposed to the environment and active exchange occurs. All arthropods are Coelomates. There are three groups of arthropods:

  1. Chelicerates – have claws for feeding, no antennae, and basic eyes. This group includes sea spiders, horseshoe crabs, ticks, mites, scorpions, and general spiders.
  2. Myriapods – This group includes millipedes and centipedes. The group inhabits terrestrial environments.
  3. Pancrustaceans – This group includes Crustaceans like crabs, lobster, and shrimp as well as Insects.


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Echinoderms are deuterostomes that inhabit marine environments and move very slowly. This group includes sea stars, sea urchins, and sea cucumbers. Although this group looks like it shows radial symmetry, it descended from bilateral ancestors. One feature unique to Echinoderms is that they have a water vascular system which help in feeding and movement. There are five groups of Echinoderms:

  1. Asteroidea – Include Sea Stars and Sea Daisies. These groups have thick arms and are powerful.
  2. Ophiuroidea – Brittle Stars which move by lashing their thin arms
  3. Echinoidea – Include Sea Urchins and Sand Dollars. They have no arms but have tube feet which allows them to be classified as echinodermata.
  4. Crinoidea – Include Sea Lilies and Feather Stars. This group has a stalk and flexible arms
  5. Holothuroidea – This group consists of Sea Cucumbers. Although they do not look like Echinoderms they have tube feet.


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