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Characteristics of Phylum Cnidaria



1.- Cnidocytes present, typically housing stinging organelles called nematocysts.


2.- Entirely aquatic, some in freshwater, but most marine.


3.-Radial symmetry or biradial symmetry around a longitudinal axis with oral and aboral ends; no definite head.


4.- Two types of individuals: polyps and medusae.


5.- Adult body two-layered (diploblastic) with epidermis and gastrodermis derived from embryonic                                            ectoderm and endoderm, respectively.


6.- Mesoglea, an extracellular matrix (“jelly”) lies between body layers; amount of mesoglea variable; mesoglea with               cells and connective tissue from ectoderm in some. 


7.- Incomplete gut called gastrovascular cavity; often branched or divided with septa.


8.- Extracellular digestion in gastrovascular cavity and intracellular digestion in gastrodermal cells.


9.- Extensible tentacles usually encircle mouth or oral region.


10.- Muscular contractions via epitheliomuscular cells, which form an outer layer of longitudinal fibers at base of                    epidermis and an inner layer of circular fibers at base of gastrodermis; modifications of plan in hydrozoan medusa          (independent ectodermal muscle fibers) and other complex cnidarians.


11.- Sense organs include well-developed statocysts (organs of balance) and ocelli (photosensitive organs); complex eyes         in members of Cubozoa.


12.- Nerve net with symmetrical and asymmetrical synapses; diffuse conduction; two nerve rings in hydrozoan                        medusae.


13.- Asexual reproduction by budding in polyps forms clones and colonies; some colonies exhibit polymorphism.


14.- Sexual reproduction by gametes in all medusae and some polyps; monoecious or dioecious; holoblastic                                indeterminate cleavage; planula larval form.


15.- No excretory or respiratory system.


16. No coelomic cavity.




Integrated principles of zoology / Cleveland P. Hickman, Jr. ... [et al.]. – 14th ed. p. cm.

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