Main Difference – Polyp vs Medusa
Polyp and medusa are the two types of body forms identified in the Cnidarians. The phylum: Cnidaria consists of four classes: Hydrozoa (Hydra, siphonophores), Scyphozoa (Jellyfish), Cubozoa (Box jellies) and Anthozoa (Sea anemones, corals, sea pens). Anthozoa is sessile. Scyphozoa and Cubozoa are free-swimming whereas Hydrozoa can be found in either freshwater or marine habitats. Both sessile and swimming members can be found in Hydrozoa. Sessile structures are called polyps while the swimming forms are called medusa. The key difference between polyp and medusa is that polyp is a fixed, cylindrical structure, representing the asexual stage and medusa is a free swimming, umbrella-like structure, representing the sexual stage.
This article studies,
1. What is Polyp
– Characteristics, Structure
2. What is Medusa
– Characteristics, Structure
3. What is the difference between Polyp and Medusa
What is a Polyp
Polyp is one of the two body forms found in Cnidarians. They are approximately cylindrical-shaped animals. Solitary polyps are attached to the substrate by their adoral end. The attachment occurs through the pedal disc which is a disk-like holdfast. Some polyps form colonies. Each polyp is connected to the other polyps in the colony either directly or indirectly. The mouth of polyps is found at the oral end. It is surrounded by a circlet of tentacles. Polyps reproduce asexually by budding.
The polyp’s body can be considered as a sac. The wall of a polyp is composed of two cell layers. The outer layer is technically considered as the ectoderm and endoderm is the inner layer. Endoderm is also called the gastroderm since the stomach is enclosed by the endoderm. A supporting layer exists between the ectoderm and the endoderm. The supporting layer is called as the mesoglea which is composed of a structureless gelatinous substance, secreted by the cell layers of the body wall. The mesoglea varies from a very thin layer to a fair thickness. Sometimes it contains skeletal elements which are formed by the cells migrated to form the ectoderm.
Since the class Anthozoa is sessile, they always compromise of polyps. In the class Hydrozoa, Hydra is considered as a polyp. Scyphozoans consist of a larval planula metamorphose as a polyp. This polyp is also called “scyphistoma”. Cubozoan planula also develops as a polyp after its settlement on a suitable environment. A sea anemone, which is an Anthozoan polyp is shown in figure 1. The polyps are simple animals, considered as living fossil which has no change for about half a billion years.
Figure 1: Sea anemone
What is Medusa
Medusa is the other body form found in Cnidarians. Medusae are aquatic animals which are considered as free-swimming soft bodies. They contain a gelatinous, umbrella-shaped bell along with the trailing tentacles. The bell pulsates to acquire propulsion and locomotion. Tentacles are used to either capture prey or defend against predators. They contain toxins in a painful sting. The mouth/anus is located at the base of the bell.
Siphonophores are the medusian animals which belongs to the class Hydrozoa. The medusa stage is dominant in class: Scyphozoa. Jellyfish can be found in the ocean, from its surface as well as from the deep seabed. Jellyfish is considered as the oldest multi-organ animal, which roamed the sea for at least 500 million years. Jellyfish has a loose network of nerves called as nerve-net located in the epidermis. Some jellyfish also have light-sensitive organs called ocelli.
The scyphistoma is developed into a stack of plate-like medusa from the larval planula of Scyphozoans. Strobilation is the pinching off and swimming away in this medusa form from the larval planula. One stage of Cubozoans’s life cycle also occurs in the medusa form. Medusae release sperms/eggs into the surrounding water during their sexual reproduction. The fertilised eggs mature into new organisms.
Figure 2: Jellyfish
Difference Between Polyp and Medusa
Movement
Polyp: Polyp is a sessile organism.
Medusa: Medussa is a free swimming organism.
Body Structure
Polyp: Body is a cylindrical structure containing a long stalk.
Medusa: Body is saucer-shaped, umbrella-like structure containing a reduced stalk.
Habitat
Polyp: It is found attached to rocks near water mark.
Medusa: It is found in open waters.
Manubrium
Polyp: Manubrium is directly upward since the body is attached to a surface.
Medusa: Manubrium hangs downward since the base is above.
Velum
Polyp: Velum is absent.
Medusa: Velum is present.
Sense Organs
Polyp: Polyp lacks sense organs.
Medusa: Statocysts are present at the bases of eight adradial tentacles.
Gastrovascular Cavity
Polyp: It is simple, and lacks radial and circular canals.
Medusa: Gastrvascular cavity is the stomach and it consist of four radial canals and one circular canal.
Gonads
Polyp: Gonads are absent.
Medusa: It consists four gonads.
Reproduction
Polyp: It reproduces asexually by budding.
Medusa: It reproduces sexually by gametes.
Role
Polyp: It is concerned with feeding, protection and asexual reproduction.
Medusa: It is concerned with sexual reproduction.
Conclusion
Most of the Cnidarian classes utilise polyp and medusa as two stages of their life cycle. Polyps are sessile and asexually reproduce by budding. But, medusa reproduces sexually by producing sperms and eggs. The key difference between polyp and medusa is that polyp is sessile whereas medusa is free-swimming.
Reference:
1. “Cnidaria”. Wikipedia, the free encyclopedia, 2017. Accessed 24 Feb 2017
2. “Polyp”. Wikipedia, the free encyclopedia, 2017. Accessed 24 Feb 2017
3. “Jellyfish”. Wikipedia, the free encyclopedia, 2017. Accessed 24 Feb 2017
Image Courtesy:
1. “Arachnanthus nocturnus”. By LASZLO ILYES (laszlo-photo) from Cleveland, Ohio, USA – Flickr (CC-By-2.0) via Commons Wikimedia
2. “Stingless jellyfish” By Riza Nugraha from Singapore, Singapore – Floating Umbrella (CC-By-2.0) via Commons Wikimedia
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