Lab 2-Cnidaria and Ctenophora

Radial symmetry refers to the fact that

any plane passing through the oral-aboral axis divides the animal into two equal halves, each a mirror image of the other

Hexacorallia -

The order Actiniaria or anemones, which you've already examined and the order Scleractinia, the true (stony or hard) corals. Please familiarize yourselves with these creatures

Octocorallia -

Tubipora or organ-pipe coral, Gorgonia or sea fans, Ptilosarcus or sea pens, Renilla or the sea pansies.

Unique organelles, called cnidae, are found in cells called

cnidoblasts

Anthozoan mouth

the mouth (the opening in the oral disc) - the mouth leads into a gullet which is a passageway leading into the gastrovascular cavity. Look for the siphonoglyph, a vertical groove at the corner of the mouth in anthozoans. Cilia in the siphonoglyph create a constant flow of water into the pharynx.

Anthozoan pedal disc (= basal disc)

the point of attachment to the substrate The dissection of the sea anemone is simple. Carefully use your scissors to cut down the middle of the specimen

The basic body plan of the cnidarians is a

sac-like structure, with an internal cavity called the gastrovascular cavity. The gastrovascular cavity has a single opening that serves as both mouth and anus, and it is often surrounded by tentacles. The body wall has an external cell layer, the epidermis (ectodermal in origin); an internal cell layer lining the gastrovascular cavity known as the gastrodermis (endodermal in origin); and a layer between the other two, called the mesoglea, which may be either cellular, or more often, acellular.

Subphylum Anthozoa -

the term "anthozoa" literally means "flowering animals" in reference to the brightly colored forms exhibited by some members of this class. They exist in the sessile polyp stage only; no medusa stage is present. The subphylum Anthozoa is a large clade whose representatives include the sea anemones and sea corals. In addition to examining the preserved specimens of sea anemones, you should examine preserved specimens of corals and some additional animals I've put out for you.

PHYLUM CTENOPHORA

(Comb Jellies) The Ctenophora are among the most beautiful of marine organisms. They are transparent, pelagic animals with bilateral symmetry superimposed on a basic radial symmetry. They are never colonial and have no sessile stage.

Members of the phyla Cnidaria and Ctenophora are considered to be more "advanced" than poriferans for two major reasons.

1. they show true tissue-level organization of body cells. 2. the adult forms are derived from two distinct embryonic germ layers, the ectoderm and the endoderm (they are diploblastic) although this point is in dispute - some workers believe muscle cells in cnidarians are mesoderm, which would make these triploblastic. Most other animal phyla are triploblastic (derived from three distinct embryonic germ layers).

Three classes of Cnidaria

: Hydrozoa, Scyphozoa, and Anthozoa as well as several Ctenophores.

Class Scyphozoa -

Animals in this class are entirely marine.

Other Hydrozoans of interest are Physalia, the Portugese man-of-war and Velella, the by-the-wind sailor.

Both have the ability to float. The man-of-war is considered dangerous to people. Velella is considered to be a single large polyp. Examine the specimens provided.

Ctenophores resemble ___________ ___________, but are not related to them

Ctenophores resemble cnidarian medusae, but are not related to them (note also that medusae evolved secondarily from polyps within the Cnidaria). Like cnidarian jellies though, they possess a branched gastrovascular system (hence in the past the Cnidaria and Ctenophora were often united in the Coelenterata), are diploblastic, and carnivorous. However they lack cnidae and have another organelle for capturing prey: sticky colloblasts.

Class Hydrozoa -

Members of this class include the genera Hydra, Obelia, and Gonionemus. Most members of this class exhibit both the polyp and medusa stages; however, Hydra exists only in the polyp form.

Important Subclasses of the Anthozoans are the ).

Octocorallia (Alcyonaria) and Hexacorralia (Zoantharia

complex and simpler Cnidarians

Pleurobrachia is a typical, if simple ctenophore; Mnemiopsis is more complex. The description here is focused on Pleurobrachia, while differences in Mnemiopsis are noted below. Note the round, biradially symmetrical body, filled with mesogloea. There are 8 longitudinal comb rows running the length of the body (Ctenophore means comb bearing). These comb rows are comprised of a series of comb plates, or ctenes, each built of large, heavy, fused, macrocilia arranged along a line. Ctenophores use their ctenes for swimming; they are the largest metazoans to move strictly by ciliary power. The ctenes in each row beat in metachronal waves and propel the organism's mouth forward through the water. Ctenes look iridescent in live animals as they diffract light because of their closely-spaced macrocilia. Ctenes are fueled by giant mitrochondria (10 μm) and keep on going even when detached from the animals. The gastrovascular system of ctenophores takes food to where it is needed. The mouth is at one end of the animal, paired (but tiny, and largely dysfunctional) anuses at the other, with pharynx and intestine between. Like Cnidarians, all ctenophores are carnivorous and may use tentacles to capture food. Peripheral gastrovascular canals arise from this central canal to the most energy-demanding body parts: the pharynx itself (paired pharyngeal canals), ctene rows (8 meridional canals), and tentacles (tentacular canals). Pleurobrachia has a pair of highly extensible tentacles with numerous, simple side branches. The tentacles are stretched out and suspended in the water column as a sticky fishing "net", by the animal swimming away from them. The side branches of the tentacles have specialized cells that harbor sticky organelles, called colloblasts that capture prey in a manner similar to some nematocysts. Only one species of ctenophore produces nematocysts. All other ctenophores have colloblast cells, which function in food capture by sticking to prey items.

more about anthozoa

The digestive tract lacks an anus; waste products are regurgitated out of the mouth and so are the sperm and eggs. The gastrovascular cavity is partitioned by curtain-like muscular membranes called mesenteries or septa, the edges of which secrete digestive enzymes. There are small openings in the septa, ostia, which allow water to pass between the internal compartments. When the longitudinal muscles of the septa contract, the animal retracts. When circular muscles around the body contract, the animal elongates. The septa often end in thin strands called acontial filaments which are loaded with nematocysts.

The life cycle of a typical cnidarian

alternates between an often-sessile polyp stage and a free-swimming medusa stage. Both stages exhibit the body plan described above; however, the polyp stage is cylindrical and attached at the aboral end to a substrate, while the medusa stage is flattened in appearance with the mouth oriented downward. In some cnidarian classes, either the polyp or the medusa stage may be reduced or completely absent.

Cnidoblasts are especially abundant on tentacles, but may be generally distributed throughout the

epidermis and gastrodermis. One common type of cnida is the nematocyst, a specialized stinging structure used for prey capture and defense.

The organisms in the phyla Cnidaria and Ctenophora are characterized by r.

radial symmetry. Terms for direction, therefore, use the mouth as a point of reference. The end of the organism which contains the mouth is the oral end; the opposite end of the animal is termed aboral.