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BIOL 111 Final Lab Notes.doc

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McGill University
Biology (Sci)
BIOL 111
Heather Roffey

LAB 7INVERTEBRATESPhylum Cnidariaamong the simplest animals only sponges are less complexcnidoGreek for nettle prickly plantall cnidarians have cnidocytesspecialized stinging cells that assist them in capture of prey and defencemost are marinegeneral body plan consists of two layers of cells surrounding a cavitythey are radially symmetricaltwo life formsopolypa generally sessile form in which the oral surface mouth is upward and the other end aboral surface is attached to a substrateomedusaa freeswimming form in which the mouth is usually downwardsome cnidarians show only one form or bothdimorphic life cyclesome have a life cycle that constantly changes between the two formsHydra class Hydrozoa is a small freshwater polyp with no medusa stage unlike other hydrozoansothe hydra attaches its basal disc foot to a substrate and extends its tentacles and body columnoif disturbed the hydra may curl up into a ballothe hydra may move in the following waysgliding on its foot inching along with its tentacles and footsomersaultingfloating on a bubble of gas secreted by the footohydras are carnivoresothey capture prey with their tentacles and restrain them with toxic stings of their nematocystothe food is transferred to the mouth partially digest in the gastrovascular cavity and then absorbed by the gastrodermal cells in which intracellular digestion occursoundigested material exits the mouthoacetic acid can cause the hydra to discharge its nematocysts from their cnidocytes which contain harpoonlike structuresohypostomethe conical elevation with the mouth opening at its tip When hydra is not actively feeding cells stick together and cover the openingotentaclesfoodcatching arms that radiate from the hypostome Tentacles contain cnidocytes for capturing prey and defenceogastrovascular cavitylocation 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 epidermisobody columncomposed of mesoglea sandwiched between two tissue layers of cellsepidermisouter layer of epithelial cellsgastrodermisinner layer of epithelial cells1 of 28omesogleaa 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 skeletonobasal disclower posterior end of the body that is used to attach to a substrate and assists in locomotioncell types in Hydraoepitheliomuscular cellscells 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 intracellularlyocnidocytesalso called cnidoblasts They are cells in the epidermis most concentrated in the tentacles and hypostome These cells contain nematocyst capsules which arebristlelike projections on the outer surface of the cnidocyte Thetriggered by cnidocils cnidocils are modified epidermal cilia Nematocysts can be either stinging entangling or adhesive this is a key way of identifying different cnidariansoneuronsnerve 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 togethersensory cellsa 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 movementogland cellscells 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 enzymesointerstitial cellscells 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 cnidocytesreproduction of HydraoHydra can reproduce both sexually or asexuallyoin sexual reproduction gametes sperm and egg unite to produce a zygoteogametes are produced in gonad which appear like a small lump in the epidermis of the body columnosperm 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 footothe ovaries are slightly larger than the testesosome are hermaphroditic Most hydra however are individual sexesosexual reproduction usually occur in the fallosperm swims to egg by entering the ovary through a hole in the epidermisothe zygote produces a hard covering and drops to the sediments and will remain dormant until spring when a polyp emergesoasexual 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 independentlyobudding usually takes 2 to 4 day to completeGonionemus class hydrozoa is a small shallow temperate marine medusait is about 25 cm in diametersome species of this genus cling to seagrasses rather than swimming2 of 28
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