Unit Five. Evolution of Animal Life
19.5. Cnidarians: Tissues Lead to Greater Specialization
All animals other than sponges have both symmetry and tissues and thus are eumetazoans. The structure of eumetazoans is much more complex than that of sponges. All eumetazoans form distinct embryonic layers. The radially symmetrical (that is, with body parts arranged around a central axis) eu- metazoans have two embryonic layers: an outer ectoderm, which gives rise to the epidermis (the outer layer of purple cells in the Phylum Facts illustration on the facing page), and an inner endoderm (the inner layer of yellow cells), which gives rise to the gastrodermis. A jellylike layer called the mesoglea (the red-colored area) forms between the epidermis and gastrodermis. These layers give rise to the basic body plan, differentiating into the many tissues of the body. No such tissues are present in sponges.
The most primitive eumetazoans to exhibit symmetry and tissues are two radially symmetrical phyla whose bodies are organized around an oral-aboral axis, like the petals of a daisy. The oral side of the animal contains the “mouth.”
Radial symmetry offers advantages to animals that either remain attached or closely associated to the surface or to animals that are free-floating. These animals don’t pass through their environment, but rather they interact with their environment on all sides. These two phyla are Cnidaria (pronounced ni-DAH-ree-ah), which includes hydra (figure 19.5a), jellyfish (figure 19.5b), corals (figure 19.5c), and sea anemones (figure 19.5d), and Ctenophora (pronounced tea-NO-fo-rah), a minor phylum that includes the comb jellies. These two phyla together are called the Radiata. The bodies of all other eumetazoans, the Bilateria, are marked by a fundamental bilateral symmetry (discussed in section 19.6). Even sea stars, which exhibit radial symmetry as adults, are bilaterally symmetrical when young.
Figure 19.5. Representative cnidarians.
(a) Hydroids are a group of cnidarians that are mostly marine and colonial. However, Hydra, shown above, is a freshwater genus whose members exist as solitary polyps. (b) Jellyfish are translucent, marine cnidarians. Together, (c) corals and (d) sea anemones comprise the largest group of cnidarians.
A major evolutionary innovation among the radiates is the extracellular digestion of food. In sponges, food trapped by a choanocyte is taken directly by endocytosis into that cell, or into a circulating amoeboid cell, where the food is digested. In radiates, digestion begins outside of cells, in a gut cavity, called the gastrovascular cavity. After the food is broken down into smaller pieces, cells lining the gut cavity will complete digestion intracellularly. Extracellular digestion is the same heterotrophic strategy pursued by fungi, except that fungi digest food outside their bodies, while animals digest it within their bodies, in a cavity. This evolutionary advance has been retained by all of the more advanced groups of animals. For the first time it became possible to digest an animal larger than oneself.
Cnidarians (phylum Cnidaria) are carnivores that capture their prey, such as fishes and shellfish, with tentacles that ring their mouths. The Phylum Facts illustration walks through the key characteristics of cnidarians, including the spaghetti-like tentacles that surround the mouth. These tentacles, and sometimes the body surface, bear stinging cells called cnidocytes, which are unique to this group and give the phylum its name. Within each cnidocyte is a small but powerful harpoon called a nematocyst, which cnidarians use to spear their prey and then draw the harpooned prey back to the tentacle containing the cnidocyte. An undischarged and a discharged nematocyst are shown enlarged in the Phylum Facts illustration. The cnid- ocyte builds up a very high internal osmotic pressure and uses it to push the nematocyst outward so explosively that the barb can penetrate the hard shell of a crab.
Cnidarians have two basic body forms: medusae, the floating form in figure 19.6, and polyps, the sessile form. Many cnidarians exist only as medusae, others only as polyps, and still others alternate between these two phases during the course of their life cycles. Figure 19.7 shows the life cycle of a cnidarian that alternates between both forms. Medusae are free-floating, gelatinous, and often umbrella-shaped forms that produce gametes. Their mouths point downward, with a ring of tentacles hanging down around the edges (hence the radial symmetry). Medusae are commonly called “jellyfish” because of their gelatinous interior or “stinging nettles” because of their nematocysts. Polyps are cylindrical, pipeshaped animals that usually attach to a rock. They also exhibit radial symmetry. The Hydra, sea anemones, and corals shown in figure 19.5 are examples of polyps. In polyps, the mouth faces away from the rock and therefore is often directed upward. For shelter and protection, corals deposit an external “skeleton” of calcium carbonate within which they live. This is the structure usually identified as coral.
Figure 19.6. The two basic body forms of cnidarians.
The medusa (top) and the polyp (bottom) are the two phases that alternate in the life cycles of many cnidarians, but several species (corals and sea anemones, for example) exist only as polyps.
Figure 19.7. The life cycle of Obelia, a marine colonial hydroid.
Polyps reproduce asexually by budding, forming colonies. They may also reproduce sexually by the formation of specialized buds that give rise to medusae, in which gametes are produced. These gametes fuse, producing zygotes that develop into planulae, which, in turn, settle down to produce polyps.
Key Learning Outcome 19.5. Cnidarians possess radial symmetry and specialized tissues and carry out extracellular digestion.