Introduction
The phylum Ctenophora, also known as comb jellies, represents a unique group of exclusively marine, diploblastic or triploblastic, biradially symmetrical organisms. They are characterized by eight rows of ciliary plates, called ctenes, which they use for locomotion. Ctenophores are significant in marine ecosystems as both predators and prey, often playing a role in controlling planktonic populations.
General Characteristics of Ctenophora
Habitat: Exclusively marine, mostly pelagic (living in open water). Found in both shallow and deep seas.
Body Symmetry: Biradial symmetry — a combination of radial and bilateral symmetry.
Germ Layers: Diploblastic or possibly triploblastic (depending on interpretation). Body wall consists of epidermis, gastrodermis, and mesoglea.
Body Structure: Transparent, gelatinous body. Lacks a skeleton. Body size ranges from a few millimeters to over 1 meter.
Locomotion: Locomotion through eight meridional rows of ciliary comb plates (ctenes) — unique among animals. Cilia beat in a coordinated manner for propulsion.
Nervous System: Simple nerve net with a statocyst at the aboral end acting as a balance organ.
Digestive System: A complete gastrovascular canal system with two anal pores. Mouth at the oral end.
Tentacles: Usually two, retractile and branched, with colloblasts (adhesive cells) instead of nematocysts for capturing prey.
Reproduction: Mostly hermaphroditic. Fertilization is external. Development is direct or with a cydippid larva.
Bioluminescence: Many species are bioluminescent, producing light in dark environments.
Classification of Ctenophora (up to Class):
Phylum Ctenophora is divided into two main classes based on body form, tentacle arrangement, and other features.
Class 1: Tentaculata
Possess two long, retractable tentacles with colloblasts.
Examples: Pleurobrachia (Sea gooseberry), Cestum (Venus’ girdle), Hormiphora
Key Features:
1. Tentacles present.
2. Includes most known species.
3. Body form may be spherical, oval, or ribbon-like.
Class 2: Nuda
Lacks tentacles entirely.
Examples: Beroe
Key Features:
a. Predatory, feeding mainly on other ctenophores.
b. Barrel-shaped body.
c. Large mouth with ciliary plates.
References
Ruppert, E. E., Fox, R. S., & Barnes, R. D. (2004). Invertebrate Zoology (7th ed.). Brooks/Cole.
Hickman, C. P., Roberts, L. S., Keen, S. L., Eisenhour, D. J., Larson, A., & I'Anson, H. (2017). Integrated Principles of Zoology (17th ed.). McGraw-Hill.
Brusca, R. C., & Brusca, G. J. (2003). Invertebrates (2nd ed.). Sinauer Associates.
Pechenik, J. A. (2015). Biology of the Invertebrates (7th ed.). McGraw-Hill.
Meglitsch, P. A., & Schram, F. R. (1991). Invertebrate Zoology. Oxford University Press.
Frequently Asked Questions (FAQs)
Q1: Are ctenophores the same as jellyfish?
No. Ctenophores are different from cnidarians (true jellyfish). They lack nematocysts and instead use colloblasts for capturing prey.
Q2: How do ctenophores move without muscles?
They use rows of ciliary plates (ctenes) for locomotion — the largest animals to do so.
Q3: What is the function of the statocyst in ctenophores?
It acts as a balance organ, helping maintain orientation in the water.
Q4: Do ctenophores have a circulatory system?
No, circulation occurs via diffusion through the body.
Q5: Why are ctenophores important ecologically?
They help control plankton populations and are food for larger marine animals.
Worksheet: Ctenophora
A. Fill in the Blanks
d. The locomotory organ in ctenophores is __________.
e. Ctenophores capture prey using __________ instead of nematocysts.
f. The larval stage in most ctenophores is called __________.
g. __________ is a bioluminescent ctenophore lacking tentacles.
h. Ctenophores exhibit __________ symmetry.
B. Match the Following
C. Short Answer Questions
1. Differentiate between Class Tentaculata and Class Nuda.
2. Describe the structure and function of ciliary comb plates.
3. Explain the role of bioluminescence in ctenophores.
D. Diagram Practice
4. Draw and label the external features of Pleurobrachia.
5. Illustrate the arrangement of ctenes in ctenophores.
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