Polyps and Medusae are the names of two body forms. A polyp is fixed and cylindrical structure which represents the asexual stage and Medusae is a free moving, umbrella-like structure which represents the sexual stage. Some animals possess both types of body forms.
What is Polyps?
Phylum Cnidarian consists of four groups (Hydrozoa, Scyphozoa, Anthozoa and Cubozoa). These four classes consist of one or two forms of body structure (Polyp and medusae). Anthozoa is a polyp, and class Hydrozoa contains both types of the body structure. They are almost cylindrical shaped animals. Solitary polyps are attached to the surface by the adoral end. The attachment with substrate occurs through the pedal disc which is a disk-like a holdfast. Some polyps live in colonies. Within colonies, polyps connect with each other directly or indirectly. The mouth of polyps is found at the oral end. The mouth is surrounding by tentacles. Polyps reproduce asexually by budding. New generated polyps form colonies which remain attached to common stalk. Such branching colonies of Obelia can be found in North American Seashores. Microscopic individual polyp forms white or yellowish colony up to 30 cm tall. The polyp’s body seems like a sac. The wall of polyp is made of two cell layers. The outer layer is called ectoderm while inner layer is called endoderm. Endoderm enclosed stomach so endoderm is called gastrodermis. A third layer is also present between ectoderm and endoderm which is called supporting layer. The third layer is called mesoglea which is composed of a structureless gelatinous substance that is secreted by the call layers of the body wall. Sometime mesoglea contains skeletal elements which are formed by the cells migrated from the outer layer. Class Anthozoa is non-motile; they always compromise of polyps. Class Hydrozoa is considered as a polyp. Class Cubozoan planula develop as polyp after settlement on a suitable environment. Animals containing polyps are simple animals, considered as living fossil which has no change for about half a billion years.
What is Medusae?
Medusae is another body form found in two classes of phylum Cnidarian. Medusae is animals of the aquatic environment which are free swimming soft bodies. They possess a gelatinous, umbrella-shaped bell along with a series of tentacles. The bell pulsates to acquire propulsion and locomotion. Tentacles are used for two purposes for capturing of preys and defending against enemies. They contain toxic material in a painful sting. The anus is located at the base of the bell. Siphonophores belong to the class Hydrozoa and medusian animals. The medusae stage is more prominent in this class. Jellyfish which is the oldest multi-organ animal found on the surface as well as from the deep seabed. It roamed the sea for about 500 million years. It has a network of nerves which is called nerve net present on the epidermis. Jellyfish also has sensitive organs called ocelli. The Scyphistoma is found into a stack of plate-like Medusae from larval planula of class. One stage of Cubozoan’s life cycle also in medusa form. Strobilation is the pinching off and swimming away in this medusae form from the larval planula. Medusae release gametes in the surrounding water during reproduction. The fertilized eggs mature into a new organism.
Polyps vs. Medusae
- Some animals are always in polyps form.
- Some animals are always in medusae form.
- Some animals possess both types of body forms.
- In animals which possess both forms, the polyp is an asexual stage, and Medusae is a sexual stage.
- Both polyps and Medusae have similar construction; a cylindrical body which is surrounding digestive cavity with a single opening called a mouth.
- Both forms consist of tentacles near the mouth which are used to capture prey.
- Body wall of both forms is composed of three layers of tissues.