Anatomy of Animals

Cnidarians

Cnidarians have radial symmetry, and there is typically a ring of tentacles surrounding the mouth region. Some cnidarians, referred to as hydroids or polyps, have a tubular shape, with the mouth region directed upward. Others, which have a bell

		shape with the mouth region directed downward, are called jellyfishes, or medusae. The polyp is adapted to a sessile life, while the medusa is adapted to a floating or free-swimming existence. At one time, both body forms may have been a part of the life cycle of all cnidarians, because today we see an alternation of generations life cycle of these two forms in certain cnidarians (see Figure 05d). In such life cycle, the polyp stage produces medusae, and the medusae, which produce eggs and
Figure 05a Cnidaria [view large image]	Figure 05b Hydra Anatomy [view large image]	sperm, disperse the species. Cnidarians are quite diversified including the Portuguese man-of-war, sea anemones, hydra and many other species.

Circulatory - Nutrient molecules are passed by diffusion to the rest of the body from the cells of the gastrodermis.
Digestive - The cells of the gastrodermis secrete digestive juices that pour into a central cavity. The enzymes begin the digestive process, which is completed within food vacuoles when small pieces of the prey are engulfed by the cells of the gastrodermis (see Figures 05a and 05b).
Exocrine - There are no endocrine glands in cnidarians. But they have specialized stinging cells called cnidocytes, from a Greek word meaning sea nettles. Within these cells are the nematocysts, which are long spirally coiled hollow threads. When the trigger of a cnidocyte is touched, the discharged thread, which sometimes contains poison, stuns either prey or enemy (see Figure 05b).
Excretory - The main opening of the cnidarians serves as a mouth and anus.
Immune - Cnidarians have primitive immune system. They use phagocytosis to eliminate attacking cells (in grafts between strains), apoptosis (cell death) plays an important part as well. In addition, there is evidence that IgG (immuno- globulins type G - a kind of antibody) related genes is involved in the response of cnidarian cells to microbial infection.
Musculo-skeletal - The body of a cnidarian is a hollow, 2-layered sac, which accounts for the former name of these organisms - coelenterate means hollow sac. The outer layer of the sac, the ectoderm, is separated from the inner layer, the endoderm, by a jellylike material called mesoglea (see Figure 05a). Each coral polyp is an individual but has a chalky skeleton that is joined to its neighbor's to form the coral reef. Hydras usually remain in one place, they may glide along on their base, or foot, or even move rapidly by means of somersaulting. Like other animals capable of locomotion, they possess both muscle and nerve cells.

		Nervous and Sensory - The nerve cells form a connecting network throughout the mesoglea known as the nerve net. The nerve net makes contact with the outer layer of cells, called the epidermis, and the inner layer of cells, called the gastrodermis. These cells contain contractile fibers. The first light-sensitive cells appeared in Cnidarians animals such as the hydras. Figure 05c illustrates the sequence of vision evolution with an insert to indicate the kind of light perception in each step.
Figure 05c Evolution of Vision [view large image]	Figure 05d Compound Eye

The box jellyfish is an amazing animal with more than 20 eye pits and many eyes very similar to ours. They seem to double for ears as well.
Hydras are among those animals that are limited to light-detecting cells spread around their bodies to pick up change in brightness. The eyespots consist of a single photoreceptor cell and a surrounding pigment cell.
In animals like flatworms, the eyespots grew inward to form cup-shaped structures to receive light from different angles.
Over time the cup became a deep chamber. As the opening to the chamber became narrower, the pinhole-eye developed. This type of eye can provide coarse images, allowing an animal to discern the shape of objects. Optic nerve to convert light energy to electric impulse also appeared in this type of eye, now found only.
Eventually the chamber opening closed over with transparent cells, which helped to prevent eye infections, creating a lens that focuses light onto the retina (a layer of photoreceptor cells). Marine snails have this type of eye, which provides the ability to see more clearly through water.
The lens evolved into a refractive lens that could change shape to focus many rays of light onto one spot on the retina. Cephalopods such as cuttlefish, squid and octopuses have eyes that are similar to those of vertebrates like humans (with small variation in the orientation of the retinal cells). Insects have compound eye with multiple facets, each with a fixed lens and crystalline cone that focuses light on the photoreceptor cells (Figure 05d).

Reproductive - In hydra, the gastrodermis contains interstitial or embryonic cells capable of becoming other types

of cells (Figure 05b). For example, they can produce the ovary and the testes and probably account for the animal's great regenerative powers. Like the sponge, a whole cnidarian can grow from a small piece. When conditions are favorable, small outgrowths, or buds, appear, pinch off, and begin to live independently. Figure 05e shows the cnidaria life cycle involving both sexual and asexual reproductions. The larva in the life cycle represents a juvenile form of the animal undergoing metamorphosis (form change). A rather controversial theory proposed

Figure 05e Cnidaria Life Cycle [view large image]

that widely separated evolutionary lineages had occasionally come together to form hybrid species. The larval and adult forms of marine invertebrates often look like entirely different organisms because that is exactly what they used to be.

One hybridisation experiment in laboratory produces star fish larvae, then sea cucumber larvae, and finally taking the form of the pluteus larvae of the paternal species - two species of Australian sea urchin. It is suggested that the theory has major potential as an evolutionary mechanism to instigate the Cambrian explosion, for example. The hybridisation theory expresses the similar idea of lateral gene transfer and the ring of life proposed for more primitive life forms.

Respiratory - The presence of the large inner cavity makes it possible for all cells to exchange gases directly with the surrounding medium. Because this function is carried out by a vascular system in more complex animals, the cavity is known as a gastrovascular cavity.