Cnidaria Dissection Guide Slide study guide
Cnidaria • Diploblastic; endoderm & ectoderm separated by mesoglea • Radial symmetry • Presence of cnidae/nematocysts in cnidocytes (stinging cells) (see photo at right) • Alternation of polyploid (polyp) and medusoid generations • Gastrovascular cavity with single mouth/anus • Tissue grade organization • Additional Classes not represented in lab: • Cubozoa: box jellies and sea wasps. • Staurozoa: stalked jellyfish. • Myxozoa: spore-producing parasites
Class Hydrozoa — Hydroids • Alternation of generations: polypoid generation usually dominant • Medusae usually small, transparent • A few (fire corals) produce coral-like calcareous skeleton • Polyps usually colonial, with interconnected coelenterons • No cells in mesoglea • Nematocysts only in epidermis (not gastrodermis) • Marine and fresh waters
Hydra with developing ovaries Hydra with ovaries and buds
Hydra Nerve Net neurite neuron
Class Anthozoa — Anemones & Corals • Lack medusoid stage • Polyps solitary or colonial • Coelenteron (gastrovascular cavity) subdivided by longitudinal folds of tissue • Tentacles in multiples of 6 (Hexacorallia) or 8 (Octocorallia) • Marine only
Subclass Hexacorallia (Zoantharia)Order Actiniaria • Anemones • Polyps large, usually with distinct stalk • Numerous tentacles • Lack calcified skeleton • Solitary or in aggregations but not truly colonies
Oral surface of the sea anemone showing the tentacles, mouth and siphonoglyphs.
Cross section through a Metridium looking towards the oral opening - shows complete and incomplete septa.
Detail of the oral opening of the sea anemone showing the position of the siphonoglyphs.
Cross section through the middle of a sea anemone- Shows complete and incomplete septa.
Cross section through a Metridium looking towards the basal disc - shows complete and incomplete septa.
Internal anatomy of the sea anemone - showing the pharynx and the acontia attached to the base of the cavity
Metridium- Cross section of the anemone, slide, showing the body wall incomplete septa and trilobed ends.
Order Scleractinia • Stony corals • Secrete calcareous skeleton • Most colonial with small polyps, though some solitary • But some solitary with large polyps
Order Alcyonacea • Soft corals • Lack rigid skeleton • Support provided by calcareous spicules • Colonies often tree-like or resemble anemone
Order Gorgonacea • Sea whips and sea fans • Small polyps form branching • Skeleton usually proteinaceous • Sometimes included within the Alcynonacea colonies
Class Scyphozoa — Jellyfish • Medusoid stage predominates; polyps inconspicuous • Thick gelatin-like mesoglea • Often pigmented
Aboral surface is concave and resembles a large sucker. This suction assists the animal in maintaining contact with the substratum. With it the jellyfish can remain in place on smooth vertical surfaces as you have observed in the aquarium.