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Phylum Cnidaria

By Levi Clancy for Student Reader on

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CnidariaHydrozoaOrder HydrodeaHydra

Habitat: Freshwater

As in most cnidarians, there are two layers: an outer epidermis and an inner gastrodermis. The mesoglea lies b/w the 2 cellular layers. The cnidocytes are located on the tentacles, each contain a nematocyst. This freshwated hydrozoan lacks a medusoid form.

CnidariaHydrozoaOrder HydroideaPlumularia

Habitat: Found attached to algae, mussels, or rocks in the low intertidal zone (marine).

Plumes 5-8cm high, usually arranged in clusters. Main stem with many hydrocladia (branches) that are joined with each "joint" having a hydrath. Feeding polyps small, usually protected by exoskeletal cups called hydrothecae.

CnidariaHydrozoaGenus ObeliaObelia

Habitat: Cosmopolitan species found from low intertidal to subtidal attached to hard substrates attached to hard substrates.

Anatomy: Polymorphic contains both gastro and gonozooids.

Common member of "fouling" community. Found on docks and pilings in calm marinas. These individuals will lend a furry and bushy appearance to submerged objects they have settled on.

CnidariaHydrozoaOrder SiphonophoraPhysalia

"Portuguese Man of War"

Habitat: Pelagic colonies found off gulf coasts and atlantic coasts of North America but not Pacific Coasts. Found on Western side of Pacific Ocean (east coast of Australia).


Habitat: Widely distributed coastal waters in all oceans (marine)

Ephyra Stage

Liberated disc "from strobula" that is young medusa. The symmetry of adult is laid down. 8 pairs of lappets, manubrium, and mouth is visible.

Scyphistoma Stage:

This stage feeds during summer and fall and produces new scyphostomes by budding. This stage arises from planula larva. In winter, it undergoes strobilization to produce pile of disks (soon to become released ephyra).

CnidariaHydrozoaOrder HydoideaTubularia

Habitat: Found attached to submerged object in a bay or harbor with a swift current (marine).

Perisarc covers entire stem, but does not continue into hydrath (gastrozooid). Hydrath has 2 circles of tentatcles (outer ring has aboral tentacles, inner ring has oral tentacles). Many gonophores grow on surface of hydranth body (hydranths are dioecious)

CnidariaHydrozoaOrder ChondrohoraVelellaHabitat: Floats at surface of offshore water in temperate and tropical oceans throughout world (marine).
CnidariaAnthozoaOrder PennatulaceaRenilla

Habitat: Common on sandflats, low intertidal, and adjacent subtidal areas (marine) **Collected daily**

Sea Pansy The heart shaped purple disk, called rachis, has a fleshy peduncle that anchors the animal in the sand. The colony, made up of many polyps, is capable of luminescing when stimulated. Prey on by nudibranchs and sand stars.


Habitat: Planktonic (marine)

Comb Jelly Body with 8 rows of ciliated plates, called comb rows made up with ctenes, which are used for locomotion. Ctenophores are the largest animals to use cilia for locomotion. A statocyst aids in orientation A voracious, gelatinous predator feeds mostly on other ctenophores, other Beroes, and salps by swallowing its prey whole.

CnidariaAnthozoaOrder ActinariaMetridium

Habitat: found on rocks, wharf pilings, and other man-made structures in the low intertidal and subtidal.

Acontia can be forced out through the mouth and weak spots in the body wall to attack other anemones such as Anthopleura elegantissima. Metridium can asexually reproduce through pedal laceration. Pedal laceration refers to when an anemone moves around breaking off pieces of pedal disk that can develop into tiny anemones. In dense clones of metridium, animals on the broader often develop “catch: tentacles which are loaded with nematocysts and can stretch 12cm or more to explore their surroundings. What do you think is the purpose of these tentacles?

CnidariaAnthozoaOrder GorgonaceaEugorgia

Habitat: Attached to rocks in depth of 24-30m (marine) **collected daily**

Purple Gorgonian Sea fan with slender purple/violet branches with white polyps. The branches form an interwoven pattern in one plane.

CnidariaAnthozoaOrder GorgonaceaLophogorgia

Habitat: In depths of about 15-60m attached to rocks (marine) **collected daily**

Red Gorgonian Sea fan with red branches and white polyps. Branches are not in a single plane. An ovulid snail lives and feeds on the branches of this gorgonian.

CnidariaAnthozoaOrder GorgonaceaMuricea

Habitat: Found on rocks and hard substrates subtidally from 12-30m.

Muricea generally orient at right angles to the flow to filter water and capture particles. As colonies grow, they form annual growth rings in their skeletons. The colony is mainly supported by an internal axial skeleton of gorgonin. Soft tissues of the skeleton are impregnated by spicules.

CnidariaAnthozoaOrder ActinariaAnthopleura

Habitat: Found on rocks and other hard substrates in the intertidal.

Colonial or solitary Wart-like tubercles on the body column are adhesive structures to which pebbles and shell fragments attach helping to reduce desiccation and provide protection from solar radiation. Acrorhagi, located under the feeding tentacles, are loaded with nematocysts. When Anthopleuera senses another anemone, it will extend the acrorhagi to do "battle" with the invading anemone. In colonies of clones, the anemones on the borders have larger acrorhagi and are called warriors.

CnidariaAnthozoaOrder ScleratiniaBalanophyllia

Habitat: Found on or under rocks from the intertidal to 10m depth.

Only true coral that lives intertidally on the California coast Bears no endosymbiotic algae in its tissues. Can trap food on tentacles or directly on the mesenteries when the mouth opens widely.

CnidariaAnthozoaOrder CorallimorphariaCorynactis

Habitat: On shaded rocks and ledges in the low intertidal to 30m depth.

Lack a calcareous skeleton The feeding tentacles have bulbous tips that contain very large nematocysts. When attacked by another anemone, the mesenteries can be extended through the mouth and have extra nematocysts to help repel the attack.

cnidocytecontains cnidocil, nematocyst capsule, operculum, nematocyst, and nucleus
nematocystconcentrated on feeding tentacles and gut, secreted by cnidocyte, released by touch and chemical stimuli, discharged w/ explosive force, used for food capture, digestion, and defense, loaded with nasty toxins, secreted into nematocyst capsule, has barbs, spines, and coiled tube
cnidocilsensory hair, triggers build up of osmotic pressure
polyptubular body shape, generally stationary, thin mesoglea, has a stalk
medusaumbrella body shape, moves, thick mesoglea, has manubrium. Both polyp and medusa have mouth, epidermis, gastrodermis
epidermislayer of living tissue that has contact with external environment
gastrodermislayer of living tissue that lines the gut
mesoglealayer of non living tissue between epidermis and gastrodermis, gives form, may contain living cells
coelenteron a.k.a. gastrovascular cavity, where digestion occurs
manubriummuscular cylinder where the mouth is attached at the end
velumshelf of tissues from the fringe of the bell towards the manubrium (but not attached!), opening of bell is restricted by length of velum, helps with speed (more force going through a smaller opening) Hydrozoan is faster than Scyphozoan b/c
gastrozooidfeeding polyp of Hydrozoans, has tentacles (not all) and mouth
gonozooidreproducing polyp of Hydrozoans
dactylozooiddefensive polyp of Hydrozoans, not all Hydrozoans have them, tentacles
hydranthin Order Hydroida, oral end of polyp (mouth & tentacle end)
hydrothecaperisarc surrounding the hydranth
thecatehas perisarc surrounding polyps
athecatedoesn't have perisarc surrounding polyps
gonophoreabortive medusae never set free; here fertilization take place (female)
planula larvawhat zygote develops into in Scyphozoans and Hydrozoans
actinula larvadevelop directly into medusae; they swim and feed; in Hydrozoans
hydrocoralOrder Hydrocorallina; colonial; secrete substantial calcareous skeleton; only in warm waters; dactylozooids esp. abundant and potent; aka fire coral; Hydrozoan
pneumatophoregas-filled (often CO) float in Order Siphonophora (Class Hydrozoa)
scyphistomacomes from non-feeding planula larva; asexual reproducing polyp form of Scyphozoans; undergo strobilization
strobiliationprocess where scyphistoma undergoes transverse fission to produce medusa
ephyrareproductive unit = immature medusa that broke away from scyphistoma.
body columnthe cylindrical portion of Anthozoan that can contract and relax against a hydrostatic skeleton; water volume is constant
oral diskat the end of the body column; contains tentacles and mouth
acontiain Metridium, inside cavity, mostly for digestion, can go out through pores to fight, thin filaments
zooxanthellaesymbiotic dinoflagellates; golden brown; in Anthopleura; give animal organic compounds 20-95% and gets metabolic wastes and protection from herbivores and residence from animal; photosynthetic; this is what makes corals able to live and thrive clear tropical waters
zoochlorellaesymbiotic green algae that does the same thing as zooxanthellae
ahermatypicnot reef-builders
brain coralscorals in which sinuous grooves lined with sclerosepta (thin raised ridges)
gorgoninpigment found in axial rod of gorgonians
sclerites(=ossicles) calcareous spicules
rachisheart part of sea pansy; contains autozooids (w/ tentacles and mouths) and siphonozooid clusters (suck in or spit out water) and covered with spicules
peduncletongue looking thing attached to sea pansy; used to stabilize sea pansy in sand (think mollusk foot)
comb rowsrun oral to aboral; comprised of ctenes; 8 in ctenophores (?)
ctenesused for swimming; partially fused, remarkably long cilia. (comb, think teeth on a comb)
colloblastssticky cells on the retractable tentacles of ctenophores; used for feeding
statocystsense organ that informs the bearer of orientation of the body to gravity; comprised of a statolith and setae
statolithsingle sphere of CaCO3; in the statocyst