Chapter 7: Cnidarians and Ctenophores

General features

— Equipped with specialized cells (cnidocytes)
— In cnidocytes, there is the nematocyst, a stinging organelle.
— Are all aquatic and mostly marine
— Radial or biracial symmetry
— Diploblastic
— 2 body types: polyps or medusae

Ecological relationships between cnidarians and other organisms

— Most abundant in shallow, marine habitats

Polyp

—The sessile, tubular form of a cnidarian with a mouth and tentacles at one end and a basal disk at the other
— Reproduce asexually by budding, fission, laceration of the pedal disc

Medusa

—A free-swimming cnidarian with a bell-shaped body and tentacles
— Mouth directed downward
— Tentacles may extend downward from rim of umbrella

Polyp Locomotion

— Hydra can move freely across a substrate by gliding on their pedal disc aided by mucous secretions

Medusae Locomotion

— Contract the bell, which expels water
— Cubuzoans can swim strongly

Life cycles

— Zygote developers into a free swimming larva
— Planula settles, and metamorphoses into a polyp
— Produces other polyps asexually, then eventually produce a free-swimming medusae
— Medusae develop into either male or female, produce gametes, fertilization occurs in open water

Planula

a free-swimming coelenterate larva with a flattened, ciliated, solid body.

Feeding and digestion

— Mouth opens into the gastrovascular cavity (stomach)
— Catch prey with tentacles, pass them to the stomach
— Gland cells discharge enzymes to begin extracellular digestion
— Nutritive-muscular cells phagocytize food particles for intracellular digestion

Body Wall

— Consists of the outer epidermis, inner gastrodermis, Layers are separated by mesoglea

Cnidocyte

a stinging cell of a cnidarian

Cnidae

Capsule-like organelles that are contained in cnidocytes.

Nematocysts

Small capsules that contain a toxin which is injected into prey or predators

Operculum

In aquatic osteichthyans, a protective bony flap that covers and protects the gills.

Cnidocil

bristle-like structure that extends from one end of a cnidocyte and functions as a trigger

Gastrodermis

inner layer of cells that serves as a lining membrane of the gastrovascular cavity of Cnidarians

Nutritive-muscular cells

phagocytize food particles and help direct material into and out of the stomach

Hydrostatic skeleton

layers of circular and longitudinal muscles, together with the water in the gastrovascular cavity, that enable movement

Mesoglea

The jellylike substance that separates the epithelial cells in a cnidarian

Nerve net

loosely organized network of nerve cells that together allow cnidarians to detect stimuli

Class Hydrozoa

— Most are marine and colonial with both polyp and media forms

Class Hydrozoa

Hydras and Portuguese man-of-war

Hydra

— a freshwater hydrozoan, atypical because it lacks a Medusa stage
— Feed on small crustaceans, insect larva, and annelids
— Asexual reproduction through budding
— In sexual reproduction, gonads are temporarily formed

Hydroid colonies (Obelia)

— are more typical
— Have a base, a stalk, and one or terminal zooids (individual polyp animals)

Class Scyphozoa

— Most of the larger jellyfishes belong to this class
— Float in open sea
— Bells vary in shape and size and are mostly composed of mesoglea
— Lack shelf-like velum found in hydrozoan medusae
— Margin of the umbrella has indentations, each bearing a pair of lappets
— Between lappets is a equilibrium sense organ called a rhopalium

Aurelia

moon jelly, typical scyphozoan

Class Staurozoa

— commonly called stauromedusans
— No Medusa stage
— Reproduce sexually
— Solitary polyp stalked with adhesive disk for attachment

Class Cubozoa

— also formerly considered schyphozoans
— Medusa form is dominant; polyp is unknown
— Umbrella is square
— At base of each tentacle is a flat blade called a pedalium
— Umbrella edge turns inward to form a velarium to increase swimming efficiency

Class Cuboza examples

Sea wasps and box jellyfish

Class Anthozoa

— "flower animals"
— Lack a Medusa stage
— All marine, in both deep and shallow water, and Mary in size
— 3 subclasses: Zoontharia, Cerianthpatharia, and Alcyonaria

Octomerous

Eight parts, specifically symmetry based on eight

Subclass Zoantharia

Sea anemones and hard coral; hexamerous (6)

Zoantharia Sea Anemones

— Polyps larger and heavier than hydrozoan polyps
— Attach to shells, rocks, timber by pedal discs
— Crown of tentacles surrounds the flat oral disc
— Siphonoglyph (ciliated groove) creates a water current directed into the pharynx

Zoantharian Corals

— Described as miniature sea anemones that live in calcareous cups they have secreted
— No pedal disc
— Secrete a limey skeletal cup with sclerosepta projecting into the polyp

Subclass Ceriantipartharia

Black coral, tube anemone; hexamerous (6)

Subclass Alcyonaria

soft corals, sea fans, sea pens; octomerous (8)

Octocorallian Corals

— Soft coral, sea pens, sea fans
— Octomerous symmetry: 8 tentacles and 8 septa
— All are colonial
— Gastrovascular cavities communicate through tubes called solenia

Photosynthetic zooxanthellae

enable corals to live in nutrient-poor marine waters.

Threats to corals reefs

— Nutrients from fertilizer and sewage threaten coral reefs with excessive algal growth
— Higher atmospheric concentrations of CO2 tends to acidify ocean water, which makes precipitation of CaCO3 by corals more difficult metabolically
— Global warming contributes to coral bleaching

Hexamerous

Symmetry based on 6's as seen in subclass Zoantharia