bio 205 chapters 6 and 7: the cnidarians and ctenophores

phylum cnidaria

completely aquatic, most are marine
approx. 20,000 species
radial symmetry
diploblastic → tissue level of organization
incomplete gut
one of two body plans

polyp

a sessile body plan that attaches to a substrate

medusa

a motile body plan that moves freely

epidermis

single cell layer covering that develops from the ectoderm germ layer, comprised of 2 cell types

gastrodermis

single cell layer lining the gastrovascular cavity (coelenteron) that develops from the endoderm germ layer, comprised of 3 cell types

nerve net

a net-like arrangement of nerve cells that develops from the ectoderm germ layer and runs from the epidermis through the mesoglea to the gastrodermis

mesoglea

an acellular thick gelatinous fluid containing collagen that sits between the epidermis and gastrodermis

epithelio-muscle cells

cells that contain actin and have slight contractility

cnidocytes

stinging cells that can paralyze prey

gland cells

cells that secrete enzymes into the gastrovascular cavity (coelenteron) to break down ingested food

nutrient cells

cells that absorb the digested nutrients from the gastrovascular cavity (coelenteron)

substitute cells

cells that are totipotent and can change into either a gland cell or a nutrient cell

nematocyst

thread-like apparatus that sits in a pool of neurotoxin when inside the cnidocyte

cnidocil

a trigger on the cnidocytes that activates the cnidocytes when touched

operculum

opening that allows for the discharge of the nematocyst

class scyphozoa

spherical medusa body plan as adults, polyp body plan as juveniles
sea jellies (jellyfish) and sea nettles
4 oral arms that hang outside the mouth
rhopalia that contain statocysts and ocelli
order semaeostomeae

oral arms

assist with moving food into the mouth of a scyphozoan

rhopalia

small flaps that hang from the margin of the medusa, contains statocysts and ocelli

statocysts

balancing “organs”

ocelli

photoreceptive eyespots

Aurelia life cycle

male medusae produce sperm and female medusae produce eggs through meiosis
1) gametes are released into the water
2) external fertilization and development
3) planula larva (ciliated) develops
4) planula larva settles out of the water column and attaches to a substrate to become a scyphistoma
5) scyphistoma undergoes strobiliation
6) scyphistoma release ephyrae (juvenile medusa) that will become adult medusa, can also undergo budding

planula larva

larval stage for all cnidarians

scyphistoma

polyp for a scyphozoan and cubozoan

strobilation

continual fission or splitting of the scyphistoma

ephyrae

juvenile medusae that are released from the scyphistoma

class cubozoa

box-shaped medusa body plan as adults, polyp body plan as juveniles
box jellies (box jellyfish)
tentacles hang from corners of medusa margin
image forming eye lenses
reproduce the same as scyphozoan, but the schyphistoma only undergoes budding

class hydrozoa

polyp body plan as adults, medusa body plan as juveniles
hydra, hydroids, portuguese man-o-war

order hydroida

exist as solitary polyps or sessile colonies of polyps
hydra are freshwater, hydroids are marine

order siphonophora

exist as colonies of polyps on threads hanging from a pneumatophore
portuguese man-o-war
3 types of polyps that hang from the threads

pneumatopore

gas chambered float that is filled with carbon monoxide and possesses a sail or crest on top

gonozooids

reproductive polyps that produce gametes

dactylozooids

stinging polyps covered with a battery of cnidocytes

gastrozooids

polyps that ingest nutrients

Obelia life cycle

consist of a colony of polyps
hydranths → feeding polyps with tentacles
gonangia → reproductive polyps, produce tiny disks that are released and become juvenile medusae
1) medusae are male or female and produce respective gametes
2) gametes are released into the water
3) external fertilization and development
4) planula larva develops and settles out for the water column and attaches to become a new hydranth polyp

hydranth

gastrozooids, feeding polyps with tentacles

gonangia

gonozooids, reproductive polyps that produce tiny disks that are released and become juvenile medusae

class anthozoa

polyp body plan throughout entire lives
soft corals, sea fan corals, stony corals, brain corals, and sea anemones

order actinaria

sea anemones → solitary polyps
siphonoglyph
acontia inside their base

siphonoglyph

a “throat-like” structure below the mouth

acontia

threads covered with cnidocytes for additional internal stinging

sea anemone life cycle

1) adult polyps produce gametes and release them into the water
2) external fertilization and development
3) planula larva develops that settles out of the water column and attach to become a new polyp

order scleractinia

stony and brain corals
exoskeleton of calcium carbonate that is secreted around and below the polyps
possess zooxanthallae

zooxanthellae

dinoflagellates that live mutualistically within coral polyps, but coral will bleach if they leave

order alcyonacea

soft corals that lack an exoskeleton

order gorgonacea

sea fan corals that produce an upright, “screen-like” exoskeleton

orders of class hydrozoa

order hydroida, order siphonophora

orders of class anthozoa

subclass hexacorallia - order actinaria, order scleractinia
subclass octocorallia - order alcyonacea, order gorgonacea

phylum ctenophora

comb jellies
radially symmetrical
diploblastic
incomplete gut, same tissues as cnidarians
statocysts is centered on top, gelatinous body is covered in comb-like rows of cilia
mesoglea
DO NOT STING
colloblasts
capable of bioluminescence

colloblasts

“sticky cells” in order to trap prey