BIOL 111 Study Guide - Final Guide: Appendicular Skeleton, Prostomium, Humerus

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16 Mar 2012
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Phylum Cnidaria
among the simplest animals (only sponges are less complex)
cnido = Greek for nettle (prickly plant)
all cnidarians have cnidocytes specialized stinging cells that assist them in capture of prey and
most are marine
general body plan consists of two layers of cells surrounding a cavity
they are radially symmetrical
two life forms:
opolyp a generally sessile form in which the oral surface (mouth) is upward and the
other end, aboral surface is attached to a substrate
omedusa – a free-swimming form in which the mouth is usually downward
some cnidarians show only one form or both (= dimorphic life cycle)
some have a life cycle that constantly changes between the two forms
Hydra (class Hydrozoa) is a small freshwater polyp with no medusa stage (unlike other
othe hydra attaches its basal disc (foot) to a substrate and extends its tentacles and body
oif disturbed, the hydra may curl up into a ball
othe hydra may move in the following ways:
gliding on its foot
inching along with its tentacles and foot
floating on a bubble of gas secreted by the foot
ohydras are carnivores
othey capture prey with their tentacles and restrain them with toxic stings of their
othe food is transferred to the mouth, partially digest
in the gastrovascular cavity, and then absorbed by the
gastrodermal cells (in which intracellular digestion
oundigested material exits the mouth
oacetic acid can cause the hydra to discharge its
nematocysts from their cnidocytes which contain
harpoon-like structures
ohypostome the conical elevation with the mouth
opening at its tip. When hydra is not actively feeding,
cells stick together and cover the opening
otentacles food-catching arms that radiate from the
hypostome. Tentacles contain cnidocytes for
capturing prey and defence
ogastrovascular cavity location where digestion and
absorption of food occurs. Some gas exchange
(respiration) also occurs in the cells lining this cavity
(gastrodermis) as well as across the epidermis
obody column composed of mesoglea sandwiched
between two tissue layers of cells:
epidermis – outer layer of epithelial cells
gastrodermis – inner layer of epithelial cells
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omesogleaa thin gelatinous layer between the epidermis and gastrodermis. Only seen as
a thin dark line in hydras but may be larger in other cnidarians. It acts as a hydrostatic
obasal disc lower posterior end of the body that is used to attach to a substrate and
assists in locomotion
cell types in Hydra :
oepitheliomuscular cells cells that act as epithelial cells by covering and lining both
surfaces and as muscle cells by having contractile fibres. One type of these cells has
longitudinal muscle fibres and is located in the epidermis. The other contains circular
muscle fibres and is located in the gastrodermis. These two types work antagonistically to
enable movement. In the gastrodermis, epitheliomuscular cells can also engulf partially
digested food particles from the gastrovascular cavity and complete digestion
ocnidocytes – also called cnidoblasts. They are cells in the epidermis most concentrated in
the tentacles and hypostome. These cells contain nematocyst capsules, which are
triggered by cnidocils, bristle-like projections on the outer surface of the cnidocyte. The
cnidocils are modified epidermal cilia. Nematocysts can be either stinging, entangling or
adhesive (this is a key way of identifying different cnidarians)
oneurons nerve cells that extend along the base of the epidermis and the gastrodermis.
These neurons form a nerve net in each tissue layer. Neurons in the mesoglea connect the
two nerve nets together
sensory cells a type of neuron located in the epidermis that is sensitive to
external stimuli and chemicals in the water. Impulses from sensory cells are
relayed to neurons of the nerve net that synapse with epitheliomuscular cells.
This results in coordinated muscular movement
ogland cells cells located in the epidermis of the basal disc that secrete a sticky
substance used in anchoring the basal disc to the substrate. There are also gland cells in
the gastrodermis that secrete digestive enzymes
ointerstitial cells cells located between the epitheliomuscular cells of the epidermis
(difficult to locate) that are like stem cells and can differentiate into other cell types such
as neurons and cnidocytes
reproduction of Hydra :
oHydra can reproduce both sexually or asexually
oin sexual reproduction, gametes (sperm and egg) unite to produce a zygote
ogametes are produced in gonad, which appear like a small lump in the epidermis of the
body column
osperm is produced in testes in the upper half of the body near the tentacles and one egg is
produced in each ovary in the lower half of the body, near the foot
othe ovaries are slightly larger than the testes
osome are hermaphroditic. Most hydra, however, are individual sexes
osexual reproduction usually occur in the fall
osperm swims to egg by entering the ovary through a hole in the epidermis
othe zygote produces a hard covering and drops to the sediments and will remain dormant
until spring when a polyp emerges
oasexual reproduction is accomplished by budding, which occurs when a new individual
grows as a bud off of a parent hydra. This new organism is an identical copy to its parent.
Eventually, the bud detaches when it has grown enough to capture its own food and live
obudding usually takes 2 to 4 day to complete
Gonionemus (class hydrozoa) is a small, shallow, temperate, marine medusa
it is about 2.5 cm in diameter
some species of this genus cling to seagrasses rather than swimming
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some species of this genus are poisonous to human because they have a powerful toxin in their
unlike other hydrozoans, Gonionemus spends most of its life in the medusa stage, which resembles
an upside down polyp, with its mouth and tentacles facing downwards
most of the body is expanded to form a dome-like structure with a thick mesoglea which acts as a
buoy and a hydrostatic skeleton
bell the dome-shaped structure made of
epidermis, gastrodermis, and mesoglea
tentacles arms hanging from the edge of the
bell, which are used for capturing prey and
attaching to objects. There are about 80+
tentacles on some species
tentacular bulbs dark swellings located at the
base of each tentacle that contain interstitial
cells, which will develop into cnidocytes to
replace discharged ones on the tentacles
adhesive pads located near the tips of the
tentacles and are used for attaching to seagrasses
or macroalgae
cnidocytes stinging cells that occur in spiral
swellings (batteries) along the length of the
tentacles. Each cnidocyte contains a nematocyst
that explosively projects a tube capable of
penetrating the tissue of its prey and injecting a
paralysing poison
veluma shelf under the edge of the bell. Epitheliomuscular cells in the velum and bells contract
to eject a jet of water downward, pushing the animal upwards. Down movements are passive.
Scyphozoan medusae lack a velum
manubrium a tube that hangs in the space inside the bell terminating with the mouth and
leading to the stomach. It is homologous to the hypostome in the Hydra.
stomach – site of extracellular digestion
mouth site of ingestion at the end of the manubrium. The edge of the mouth is ringed by fleshy
projections called oral lobes. These structures assist with ingestion. Indigestible food is eliminated
through the mouth, as well
radial canals – four canals radiating out of the manubrium that are extensions of the stomach
ring canals connecting with the ends of the radial canals and running around the edge of the
bell. Along with the stomach, the canals form a gastrovascular system to partially digest and
circulate food throughout the body. Food particles can be engulfed by gastrodermal cells and
digestion is then completed intracellularly
statocysts – small swellings between the bases of the tentacles that contain a stone suspended on a
flexible stalk. The pressure of the stone against the cells lining the cavity of the statocyst provides
the basis for orientation with respect to gravity. Statocysts are thus sensory structures that perceive
the direction of gravity
gonads – site of gamete production. There are four gonads that hang in the bell
a fertilized egg quickly develops into a planula a ciliated larva. The free-swimming larva soon
attaches to a substrate and becomes a polyp. The polyp reproduce asexually by budding and then
each polyp produces medusa buds that grow into adult medusae
Phylum Annelida
annelida means “little rings”
this phylum contains earthworms, tubeworms, and leeches
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