8.28.2 Lesson: Worms and Mollusks

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Worms and Mollusks


H.1 – Identify the characteristics of Kingdom Animalia and its various phyla.

H.2 – Identify the internal and external structures of earthworms and perform further examination through a virtual lab.



Describes the presence of a head/brain region on the body of an organism

Eye Spot

Pigmented cell that responds to light and chemicals


Organism that contains both male and female reproductive organs

Hydrostatic Skeleton

Skeleton that maintains its form by the pressure of internal fluid


Structure of mollusks used for protection of internal organs and for gas exchange


Reproductive segments of a tapeworm that contain eggs and sperm


The head-like segment of a tapeworm that has suckers and hooks for attachment


Membrane that separates the segments of annelids


The next group of invertebrates that we will explore is composed of various types of worms. Worms have bilateral symmetry and exhibit cephalization, meaning that they have an identifiable head region. The head area of the organism generally contains sensory organs and/or nerve cells.

Worms can be divided into three phyla: Platyhelminthes, Nematoda, and Annelida.

Phylum Platyhelminthes

Members of phylum Platyhelminthes are flatworms

Flatworms have three main tissue layers: the endoderm, mesoderm, and ectoderm. Unlike sponges and cnidarians, there is no space between the layers.

Flatworms are also characterized by an incomplete digestive system, meaning that there is only one location for entry/exit. Nutrients enter the organism and wastes exit the organism all through a single opening.

The bodies of flatworms are flattened and lack segmentation. Their skeleton is hydrostatic and is composed of a water-filled cavity that has muscles wrapped around it. The muscles squeeze the cavity, causing the worm to move due to increased water pressure.

Members of this phylum, such as tapeworms, can be parasitic. Other members, such as planarians (described below), can live freely.

  • Class Turbellaria: Planarians
    • Planarians are the free-living members of phylum Platyhelminthes. Simply put, they do not require a host to survive and reproduce. The image below shows a planarian. The head is the area in the lower right-hand corner. As you can see, the organism has eye spots. These structures do not see in the same way that human eyes can, rather, they only respond to light and chemicals. The auricles, which are the triangular points on the sides of the head region, are sensitive to chemicals and touch. Nerves run down the length of the body structure. 

    • Planarians can reproduce sexually or asexually. These organisms are hermaphroditic, meaning that they possess both male and female reproductive organs. Only one set of organs is used at a time. For asexual reproduction, the planarian can regenerate if split into pieces.
  • Class Cestoda: Tapeworms
    • Tapeworms are the parasitic members of phylum Platyhelminthes. The tapeworm attaches to a host using the hooks and suckers on its head, collectively called the scolex.
    • These organisms do not contain sensory organs nor do they possess a true digestive system; instead, they reside in the digestive systems of their hosts and absorb food through their body walls.
    • Tapeworms are hermaphrodites and have a reproductive system composed of segments called proglottids, each of which is full of eggs and sperm.


  • Class Trematoda: Flukes
    • Another group of parasitic members of phylum Platyhelminthes includes flukes. These small flatworms, like tapeworms, feed off of the body of a host. One example is a worm known as the “sheep liver fluke” that attacks sheep and cattle. Their life cycle begins when a fluke’s eggs are eaten by pond snails. Once the eggs hatch inside the snail’s body, the larvae leave the snail. Once a larva is eaten by a sheep, it travels to the sheep’s liver where it develops into an adult fluke. Humans can also be infected with a fluke if raw or undercooked meat is consumed. Below is a diagram of how humans can become infected if improperly prepared fish is consumed.


Phylum Nematoda

The next phylum is composed of roundworms, or worms without segmentation. These organisms have a hydrostatic skeleton and the muscles in the bodies of these worms run lengthwise, allowing for their snake-like movement. Roundworms have a complete digestive system with two openings: a mouth for the entry of food particles and an anus for the exit of waste. Since they do not have a circulatory system, they rely on diffusion for transportation of nutrients and wastes. Some of these worms are hermaphroditic while other species have separate male and female organs.

These worms can be found in parasitic forms as well as free-living forms. Hookworms, for example, are an example of parasitic roundworms that can infect humans. The diagram below depicts the life cycle of a parasitic roundworm that has a human host.


Phylum Annelida

The next type of worm is segmented and is likely the version of the organism that you think of when you hear the word “worm.” They are bilateral and exhibit segmentation on both the inside and outside of their bodies. The number of segments can range anywhere from 50 to 500! Each of the segments is separated by a membrane called the septum. The septa aid in the movement, flexibility and organ separation of the worm. Muscles are found both in circular forms and in forms that run length-wise down the body.


Other characteristics include: the possession of a circulatory system and digestive system, single-sex forms or hermaphroditic forms, and the ability to survive in both aquatic and terrestrial environments. The three classes of annelids are marine worms (Class Polychaeta), earthworms (Class Oligochaeta) and leeches (Class Hirudinea). Leeches have suckers to attach to other organisms for feeding purposes.


Another large group of invertebrates is Phylum Mollusca, or the mollusks. All members of this phylum have soft bodies, a muscular “foot” and a mantle. The mantle serves as protection for internal organs and as a respiratory chamber for these animals. Examples include snails, slugs, clams, oysters, squids, and octopi.

Class Gastropoda

Snails and slugs are considered gastropods. These organisms are identified by a distinct head region and a “stomach foot,” meaning that they move their stomachs in a slow motion.

Gastropods are scavengers, eating anything that is available. The food is broken down as it travels through a complete digestive system. The circulatory system, however, is incomplete. While gastropods have a heart, the system does not form one continuous loop and blood vessels remain open-ended.

These organisms can be aquatic or terrestrial. The species will possess gills if it lives in an aquatic environment and it will have a simplistic version of a lung if it lives in a terrestrial environment.


Class Pelycypoda

The next class of mollusks includes bivalves such as clams, mussels, scallops, and oysters. A bivalve is an organism with a body that is compressed and enclosed by a shell that consists of two hinged parts. Thus, these animals have bilateral symmetry but do not have cephalization. The image below shows the inside of a pearl oyster.


The adult form is sessile and relies on filter-feeding methods to obtain nutrition. They have a simple nervous system and an open circulatory system.

Class Cephalopoda

Mollusks that have tentacles, which are considered modified feet, are called cephalopods. These are larger organisms that have a brain and a sensory system. They prove themselves as strong predators through their hunting methods.

Other features of their bodies include the presence of gills for gas exchange and a closed circulatory system. This class includes animals such as squids and octopi.



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