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Lab 6 - Exam Review (Phylum Cnidarian).docx

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Biology (Sci)
BIOL 111
Suzanne Gray

LAB 6 – INVERTEBRATES I Phylum Cnidarian  Simplest of animals  Contain stinging cells called cnidocytes that assist in the capture of prey and defense  Generally marine (i.e. jellyfish and corals)  General body plan consists of two layers of cells surrounding a central cavity (radially symmetry) Two General Forms: 1. Polyp: fairly sessile, in which the oral surface (mouth) is upward and the aboral surface is attached to the substrate 2. Medusa: free swimming form in which the mouth is usually downward **If organism has both forms in their life cycle, they are called dimorphic. Generally, they have life cycles that alternate between a motile sexual medusa stage and a sessile asexual polyp stage. Phylum Cnidarian  Class Hydrozoa  Hydra:  Small polyp with no medusa stage Movement:  Hydra attaches its foot (basal disc) to the bottom of the dish and extends its tentacles and body column  Move by gliding on its foot, inching along with its tentacles and foot, by somersaulting from foot, to tentacles, to foot or by floating on a bubble of gas secreted by the foot Feeding Behavior:  Carnivores  Capture prey by using their tentacles and subdue them with toxic stings of their nematocysts.  Food is transferred to their mouth, partially digested in the gastrovascular cavity, and then absorbed by gastrodermal cells. There intracellular digestion occurs. Undigested material is voided through the mouth.  Do Hydra actively move to capture its prey or does it sit and wait for the prey to bump into its tentacles? For the most part, they remain attached to their substrate and let their tentacles drift with the water currents. If a prey item (e.g., a zooplankton species) brushes into one of the tentacles, the hydra quickly grabs onto it with the other tentacles subduing it with poisonous threads/filaments from its nematocysts. Watch carefully to see if this description fits the feeding behavior of your hydra. LAB 6 – INVERTEBRATES I  How does radial symmetry of your hydra assist them with feeding? Most cnidarians are opportunistic feeders and capture prey as they bump into their tentacles, rather than stalking or chasing after its prey. Having sensory and feeding structures (tentacles) radiating from their body increases their abilities to sense and capture prey that may be coming at them from any direction. This differs from animals that go after their prey. In this case, being bilaterally symmetrical is more beneficial because the sensory structures are concentrated in one area, which is moving in one direction.  Why do we add acetic acid (vinegar) to the Hydra? The acid will induce the Hydra to discharge their nematocysts from within their cnidocytes. Structures:  Hypostome: o Conical elevation with the mouth opening its tip o Acts as the entrance (mouth) and exit (anus)  Tentacles: o Food catching arms that radiate from the hypostome o Contain specialized stinging cells called cnidocytes used to capture and defend  Gastrovascular cavity: o Where digestion and absorption of food occurs o Gas exchange (respiration) may also occur in the gastrodermis (cells lining the cavity) as well as across the epidermis  Body Column: o Composed of mesoglea (thin, gelatinous layer between the epidermis and gastrodermis, visible as a thin dark line, acts as a hydrostatic skeleton) sandwiched between two tissue layers of cells:  Epidermis: outer layer of epithelial cells  Gastrodermis: inner layer of epithelial cells  Basal Disc: o Foot o Assists in locomotion o Used to attach to the substrate Cell Types:  Epitheliomuscular cells: o Act as epithelial cells by covering and lining both surfaces o Act as muscle cells by having contractile muscle fibers o Longitudinal muscle fibers are located in the epidermis o Circular muscle fibers are located in the gastrodermis (here cells are able to engulf partially digested food particles from the gastrovascular cavity and complete digestion intracellularly) LAB 6 – INVERTEBRATES I o Work to enable movement  What is the probable function of ciliated cells in the gastrodermis? They, along with the muscular contractions of the cells in the gastrodermis, create water currents, which circulate the food throughout the gastrovascular cavity. This increases the distribution of nutrients to all the cells. Gas exchange also occurs in the gastrodermis, so water currents help not only to distribute food but also oxygen and to remove wastes.  Cnidocytes: o Located in the epidermis o Most concentrated in the tentacles and hypostome o Contains the nematocyst capsules (which are triggered by cnidocils  bristle-like projections on the outer surface of the cnidocyte) o Types of nematocysts: stinging, entangling and adhesive  Neurons: o Nerve cells that extend along the base of the epidermis and gastrodermis o Form a nerve net in each tissue layer o Neurons running through the mesoglea connect the two nerve nets together o Sensory cells: type of neuron located in the epidermis  Sensitive to touch or chemicals in water  Impulses from sensory cells are relayed to neurons of the nerve net that synapse with epitheliomuscular cells  results in coordinated muscular movement  Gland cells: o Located in the epidermis of the basal disc o Secrete a sticky substance used in anchoring the basal disc to the substrate o Also found in the gastrodermis, where they are used to secrete digestive enzymes  Interstitial cells: o Located between the epitheliomuscular cells of the epidermis o Totipotent cells (act like stem c
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