General Characteristics of CNIDARIA

General Characteristics of CNIDARIA

• Radially symmetrical, facilitating food capture from all directions.
• Diploblastic structure with epidermis and gastrodermis separated by mesoglea.
• Single gastrovascular cavity functioning as both mouth and anus.
• Absence of a head, brain, and central nervous system; features a simple nerve net.
• Sense organs include statocysts and ocelli.
• Skeletal structures may be horny or calcareous, can be internal or external.
• No specialized structures for respiration, excretion, or transport (except in some jellyfish).
• Unique cnidae in cnidocytes, with nematocysts being the most common type.
• Exhibits polymorphism, existing as solitary or colonial forms (polyp or medusa).
• Four classes identified: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa.

Class HYDROZOA

• Enteron is not partitioned by mesenteries.
• Tentacles of polyps typically solid, and mesoglea is acellular.
• Cnidocytes localized only in the epidermis.
• External chitinous skeleton predominates.
• Hydromedusa are smaller and serve as gamete producers with ectodermal gametes.
• Notable genera include Hydra, Gonionemus, and Obelia.

Class SCYPHOZOA

• Medusa form is dominant; differs from hydromedusa due to the absence of velum (acraspedote).
• Tentacles are short and may be notched along the margin.
• Mesoglea contains amoeboid mesenchyme cells.
• Cnidocytes present in both gastrodermis and epidermis, with gastrodermal gametes.

Class CUBOZOA (Box Jelly)

• Medusa is square-shaped with tentacles hanging from the corners.
• Known for strong swimming abilities and being voracious fish predators.
• Possess advanced image-forming eyes for navigation.
• Stings from some species (e.g., Chironex) can be lethal to humans.

Class ANTHOZOA (Sea Anemones and Coral)

• Only polyp form is predominant, exhibiting limited mobility.
• Oral end expands into a disc with hollow tentacles encircling a slit-like mouth.
• Gastrovascular cavity is compartmentalized by mesenteries, which contain cnidocytes and gonads.
• Mesoglea features amoeboid mesenchyme cells, with a pharynx leading from the mouth.

Class CTENOPHORA (Comb Jelly)

• Features biradial symmetry and is transparent, fragile, acoelomate, and luminescent.
• Bears eight rows of comb-like cilia for movement.
• Largest animals moving via ciliary actions, forming musculature from gelatinous mesenchyme.
• Lacks attached sessile life stages and is typically monomorphic.
• Primarily hermaphroditic, developing through a cydippid larval stage.
• Possesses a complete gut; pharynx leads to branching gastrovascular canal system with small anal pores.
• Tentacles equipped with adhesive cells called colloblasts for trapping prey, lacking stinging nematocysts.