Difference Between Cnidaria and Ctenophora

Main Difference – Cnidaria vs Ctenophora

Cnidaria and Ctenophora are two phyla containing coelenterates with a hollow gut. Both cnidarians and ctenophores are diploblastic animals. The internal body cavity serves as the gut. The gut is covered by a tissue called gastroderm. A single opening serves as both mouth and the anus in cnidarians. However, ctenophores contain a complete digestive system. Tentacles occur around the mouth of both cnidarians and ctenophores. Hydra and jellyfish are cnidarians. The comb jellies are ctenophores. The cnidarians can be either sessile or mobile. But, ctenophores are always mobile. The main difference between Cnidaria and Ctenophora is that Cnidaria exhibits radial symmetry whereas Ctenophora exhibits biradial symmetry. Both cnidarians and ctenophores possess sense organs such as statocytes and ocelli. 

Key Areas Covered

1. What is Cnidaria
      – Definition, Characteristics
2. What is Ctenophora
      – Definition, Characteristics
3. What are the Similarities Between Cnidaria and Ctenophora
      – Outline of Common Features
4. What is the Difference Between Cnidaria and Ctenophora
      – Comparison of Key Differences

Key Terms: Biradial Symmetry, Ceolentrates, Cnidaria, Ctenophora, Diploblastic, Medusa, Mesoglia, Polyp, Radial Symmetry, Statocyte, Tentacles

What is Cnidaria

Cnidaria is a phylum, which contains invertebrate animals with specialized stinging structures in the tentacles that surround the mouth. Jellyfish, coral, Hydra, sea anemone, and sea pens are the examples of cnidarians. The body type of cnidarians can be two types: a polyp or a medusa. Polypoid cnidarians contain tentacles around their mouth, which faces up. The base of the body is attached to a surface. The sea anemone, Hydra, and coral are polypoid cnidarians. Medusoids are free-swimming animals, whose mouth faced down. Therefore, their tentacles also face down. The jellyfish is a medusoid cnidarian. All cnidarians are radially symmetric. The cnidarians are also called coelenterates since their body cavity is a hollow gut. They contain an incomplete digestive system and the mouth also serves as the anus. The cnidarians are diploblastic animals; hence, their body wall is made up of two cell layers: epidermis and the gastroderm. In between those two cell layers, a gel-like substance is present, which is called mesoglea.

A sea anemone is shown in figure 1. 

Figure 1: Sea Anemone

Cnidarians consist of stinging cells in their tentacles, which are used for feeding as well as for defense. The stinging cells are also called cnidocytes. The stinging structure is made up of a hollow thread-like structure, which contains barbs inside it. The thread can unfurl to the outside, wrap around the pray, and inject toxins. Cnidarians live in freshwater and marine water. The asexual reproduction of cnidarians occurs by budding. The sexual reproduction occurs by spawning in which the sperms and eggs are released to the water columns to produce free larvae.

The five classes of cnidarians are Anthozoa (corals and sea anemones), Hydrozoa (Hydra and Obelia), Cubozoa (box jellyfish), Scyphozoa (jellyfish), and Staurozoa (stalked jellyfish).

What is Ctenophora

Ctenophora is a phylum of coelenterates, which consists of comb jellies. Ctenophores are exclusively marine animals that can be mostly identified in warmer seas. Typically, they float around the surface of the water. However, some ctenophores live in 3000 meters deep sea. Ctenophores have a pear-shaped body with a 5-20 mm diameter. They exhibit a glass transparency. Their mouth occurs in the oral pole of the body while the sensory organ called statocyte is found in the aboral pole. Statocyte identifies the gravity. Two long tentacles are attached to the narrower end. The body surface of the ctenophores contains eight meridional bands in equal distances. These bands are called swimming plates. Each plate consists of transversely arranged comb-like structures in rows. The com-like structures consist of narrow plates, fraying from the outer ends. The constant movement of the frayed plates propels the animal through the water.

Ctenophores are considered to contain a complete digestive tract since their excretion occurs through minute apparatus called excretory pores.

Figure 2: Comb Jelly

Ctenophores are hermaphrodites. The mature eggs and sperms are released to the external environment through the mouth for the fertilization. The two classes of ctenophores are Tentaculata and Nuda. A comb jelly is shown in figure 2. 

Similarities Between Cnidaria and Ctenophora

• Both Cnidaria and Ctenophora consist of coelenterate animals with a hollow gut.
• Both Cnidaria and Ctenophora contain a mouth, which is surrounded by tentacles.
• Both cnidarians and ctenophores are diploblastic animals.
• Both cnidarians and ctenophores contain mobile organisms.
• Both cnidarians and ctenophores exhibit external fertilization.

Difference Between Cnidaria and Ctenophora

Definition

Cnidaria: Cnidaria refers to a phylum, which contains invertebrate animals with specialized stinging structures in the tentacles that surround the mouth.

Ctenophora: Ctenophora refers to a phylum of coelenterates, which consists of comb jellies.

Symmetry

Cnidaria: Cnidarians exhibit radial symmetry.

Ctenophora: Ctenophores exhibit biradial symmetry.

Diversity

Cnidaria: Cnidarians are highly diversified animals.

Ctenophora: Ctenophores show less diversity.

Habitat

Cnidaria: Cnidarians live in both freshwater and marine habitats.

Ctenophora: Ctenophores live exclusively in marine habitats.

Alteration of Generations

Cnidaria: The Cnidarians show alteration of generations. 

Ctenophora: The Ctenophora does not show alteration of generations.

Polyp/Medusa

Cnidaria: Cnidarians possess both polyp and medusa stages.

Ctenophora: Ctenophores only possess the medusa stage.

Sessile/Swimming

Cnidaria: Cnidarians are sessile or free-swimming.

Ctenophora: Ctenophores swim by the comb plates.

Digestive System

Cnidaria: Cnidarians contain an incomplete digestive system.

Ctenophora: Ctenophores contain a complete digestive system.

Unisexual Animals/Hermaphrodites

Cnidaria: Cnidarians can be either unisexual or hermaphrodites.

Ctenophores: Ctenophores are hermaphrodites.

Classes

Cnidaria: The five classes of cnidarians are Anthozoa, Hydrozoa, Cubozoa, Scyphozoa, and Staurozoa.

Ctenophora: The two classes of ctenophores are Tentaculata and Nuda.

Examples

Cnidaria: Jellyfish, corals, Hydra, and sea anemones are cnidarians.

Ctenophora: Comb jellies are the examples of ctenophores.

Conclusion

Cnidaria and Ctenophora are two types of phyla composed of coelenterates. Cnidarians live in both freshwater and marine habitats while ctenophores only live in the sea. Cnidarians are diversified animals than ctenophores. Both cnidarians and ctenophores are diploblastic animals. Cnidarians exhibit radial symmetry whereas ctenophores exhibit biradial symmetry. Both contain tentacles, surrounding their mouth. The main difference between cnidarians and ctenophores is their body symmetry.  

Reference:

1. Kennedy, Jennifer. “Fascinating Facts About Cnidarians.” ThoughtCo, Available here. Accessed 25 Sept. 2017.
2. “Phylum Ctenophora: Features, Characters and Other Details.” Biology Discussion, 2 May 2016, Available here. Accessed 25 Sept. 2017.

Image Courtesy:

1. “Sea Anemone” by Jeff Kubina (CC BY-SA 2.0) via Flickr
2. “Comb jelly” by Javier Kohen (CC BY-SA 2.0) via Flickr