Cnidarians
Cnidarians are the simplest organisms that have attained a tissue level of organization. They consist of little more than an outer covering (the epidermis) and an inner gastrodermis with a non-cellular mesoglea sandwiched between the two (Fig 1). Mucus-secreting cells cover the animal with a protective slime. A single opening serves as the entrance to the gastrovascular cavity, within which prey is digested. Lacking an anus, any undigested material is ejected through the mouth. With most of the living cells in direct contact with the water, specialized excretory, circulatory, and respiratory are not needed. Nonetheless, the nervous and sensory systems are reasonably well-developed and many cnidarians are capable of quite complex behavior. All members of this phylum possess stinging structures called nematocysts housed in specialized cells (cnidocytes). Cnidocytes are especially concentrated on the tentacles, but are also found scattered over the epidermis and gastrodermis. In some species the poison contained in the barbs of the nematocysts is powerful enough to harm humans (most notably, those of the Portuguese man-of-war, Physalia). Most, however, don't penetrate the skin or if they do they cause only slight irritation.
Class Hydrozoa. This class includes creatures such as Hydra, Obelia, and Goneonemus. The fire corals and some relatives also belong to this class.
Figure 1. Longitudinal section of Hydra. Both prey and waste move through the stoma. Note that the entire gastrovascular cavity extends through each tentacle.
Two body forms are characteristic of this phylum (sometimes within the same species). The polyp is cylindrical with tentacles at one end (Hydra is built on this plan; Fig 2). Polyps are often sessile (non-motile and attached to the substratum), but some are capable of locomotion. The medusa body plan (Fig 2) is the form commonly called a "jellyfish". The mesoglea is considerably thicker than that of the polyps and imparts the jelly-like consistency to the animal. Medusae swim by undulations of their bell. The orientation of most species is mouth-side (oral surface) down (with the aboral surface up). Some cnidarians exist only in the polypoid or the medusoid form, while others alternate between the two body plans. Both polypoid and medusoid forms are radially symmetrical.
Figure 2. The colonial hydrozoan Obelia. Both polypoid and medusoid forms are shown.
Those species that alternate between the two body plans are said to have alternation of generations. Polyps usually reproduce through asexual means (budding and fragmentation) while medusae reproduce sexually. Fertilization is external and both dioecious and monoecios forms are found (depending on the species). The life cycles of a cnidarian is depicted in fig 2.
There are four classes within the phylum Cnidaria:
Class Hydrozoa. This class includes creatures such as Hydra, Obelia, and Goneonemus. The fire corals and some relatives also belong to this class.
Class Scyphozoa. The most commonly-seen example of this class is Aurelia.
Class Anthozoa. This class includes the sea anemones, true corals, and soft corals.
Class Cubomedusa. This class has cube-shapes bells as medusa. Some of these are the most venomous of all sea creatures. There's a picture here along with the aftermath of an attack here.
Figure 3. External Structure of and adult hydrozoan, Gonionemus
Members of the class Hydrozoa have unpartitioned guts as both polyps and medusa. As medusae they have a shelf-like structure called the vellum at the base of the bell
(Figure 3).
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 Class Hydrozoa: Hydra, Detail showing the epitheliomuscular and nutritive epithelio layers of the body wall cnodocytes are visible on the outer surface. |
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 Class Hydrozoa: Physalia |
 Class Hydrozoa: Physalia, detailed view of the tentacles of the Portuguese man-o-war. |
 Class Hydrozoa: Physalia gas bag and tentacles (left and middle). Welts from a man-of-war (right) |
 Class Hydrozoa: Medusa stage of an the hydrozoan Obelia. The tentacles around the bell margin, mouth and gonads are visible. |
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 Class Hydrozoa: Goniocnemus, View of a hydrozoan medusa showing the tentacles, mouth and gonads. |
 Class Hydrozoa: Goniocnemus tentacles with nematocyst batteries and adhesive pads. |
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Class Scyphozoa. The most commonly-seen example of this class is Aurelia.
Figure 3.
Aurelia Life Cycle
Aurelia is found in Class Scyphozoa. Scyphozoans can be recognized by a gut partitioned into four parts as well as the lack of a vellum in the medusa stage (Figure 3). The RHOPALIA (at the margin of the bell) are multi-purpose sensory organs (they detect light, provide a sense of gravity, and taste chemicals dissolved in the water). Note that the gonads associated with the gastric pouches. The life cycle of most schyphozoans is more complex than that of the hydrozoans. It begins with the PLANULA LARVAE (Fig 3); formed sexually by the fusion of eggs and sperm from mature jellyfish). The planula is a flattened ciliated larva. They are free-swimming for a short period and then settle on the substratum where they METAMORPHOSE into a polypoid larva called a SCYPHOSTOMA. Scyphostoma feed very much as do Hydra and reproduce through asexual budding. They eventually undergo multiple transverse fissions through a process called STROBILIZATION. The resulting STROBILA sheds immature jellyfish (EPHYRA) at the oral (tentacular) end. Ephyra eventually mature into adult jellyfish. Like most scyphozoans, Aurelia are dioecious.
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 Class Scyphozoa: Aurelia, rhopallium. |
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Jellyfish Swimming #0 | Jellyfish Swimming #1 | Giant Jellyfish More is here |
Class Anthozoa. This class includes the sea anemones, true corals, and soft corals.
Figure 4 Metridium Internal Structure
Members of the class Anthozoa include most corals and the sea anemones. They can be distinguished from the other two classes in that their gut is partitioned into more than four sections and that they lack alternation of generations. CIRCULAR MUSCLES squeeze against water in the interior cavity (which serves as a HYDROSTATIC SKELETON) resulting in a lengthening (and thinning) of the column. RETRACTOR MUSCLES (on a longitudinal axis) make the column short and fat when contracted. MESENTERAL PERFORATIONS equalize the flow of water within the animal (water flow would otherwise be restricted by the MESENTERY extending from the outer wall to the PHARYNX. ACONTIAL and MESENTERAL filaments produce digestive enzymes and aid in the absorption of nutrients.
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 Class Anthozoa: Clown fish bathing in an anemone |
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Class Cubomedusa. This class has cube-shapes bells as medusa. Some of these are the most venomous of all sea creatures.
Information on the Cubomedusae can be found here
 Class Cubomedusae |
 Aftermath of an attack |
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