CONCEPTS IN BIOLOGY
PART V. THE ORIGIN AND CLASSIFICATION OF LIFE
23. The Animal Kingdom
23.6. Primitive Marine Animals
Members of the Porifiera (sponges) and Cnidaria (jellyfish, corals, and relatives) are the simplest multicellular animals. Fossils of these kinds of animals have been found that date to about 580 million years ago. Nearly all of the animals in these groups are found in saltwater environments. There are a few freshwater species.
Even though sponges are classified as multicellular, in many ways they are similar to colonial protozoa. In particular, a group of flagellated protozoa known as choanoflagellates have a structure that is very similar to the flagellated cells (called choanocytes) found in sponges. Sponges have two layers of cells, with the choanocytes on the inside and flat, platelike cells (pinacocytes) on the outside. There are also a few other kinds of cells present. There is a jellylike layer between the two kinds of cells. The cells of sponges are rather loosely connected and can move about, and some even change their function. Because of this loosely defined organization, sponges are not considered to have tissues, which are groups of cells with specific functions.
All adult sponges are benthic, sessile (permanently attached) filter feeders. The flagellated choanocytes cause a current of water to circulate through the organism. All the cells engulf entire food particles directly from the water just as most protozoa do (figure 23.10). Although some sponges are radially symmetrical, most sponges are asymmetrical. Sponges have structures, called spicules, in a jellylike material between the two layers of cells. Some species have spicules of calcium carbonate, others of silicon dioxide, and some of a protein material. It is the skeletons composed of protein spicules that are used as bath sponges. (Actually, most sponges we buy are manufactured from various forms of plastic.)
FIGURE 23.10. Sponge Structure and Function
(a) Sponges consist of an outer epidermal layer and an inner layer of flagellated cells called choanocytes. These flagella create a current, which brings water in through the openings formed by the porocyte cells, and propels it out through the osculum. The current brings food and oxygen to the inner layer of the cells. The food is filtered from the water as it passes through the animal. (b) Some sponges have a “vase shape”; however, (c) most sponges are asymmetrical and have no distinct form.
Asexual reproduction in sponges can occur by fragmentation. Wave action may tear off a part of a sponge, which eventually settles down, attaches itself, and begins to grow. Sponges also reproduce by budding, a type of asexual reproduction in which the new organism is an outgrowth of the parent. Sexual reproduction involves the release of sperm, which are carried to adjacent sponges where fertilization takes place. The fertilized egg develops into a free-swimming, flagellated larval stage. The larva swims in the plankton and eventually settles to the bottom, attaches, and grows into an adult sponge.
Cnidaria—Jellyfish, Corals, and Sea Anemones
Members of the Cnidaria (pronounced nid'-air-e-ah) include the jellyfish, corals, and sea anemones. They are diploblastic with an outer epidermis and an inner layer that forms digestive cavity. Because the cells of these two layers are specialized, the cnidarians are considered to have a tissue level of development. There is a jellylike material between the two layers of cells; it is very thick in jellyfishes. Cnidarians show radial symmetry, and all species have a single opening leading into a saclike digestive cavity. Surrounding the opening is a series of tentacles (figure 23.11). These long, flexible, armlike tentacles have specialized cells that produce structures called nematocysts, which can sting and paralyze small organisms. Nematocysts are unique to the Cnidaria. The cells that produce the nematocysts are known as cnidocytes and are responsible for the name of the group—Cnidaria. Even though they are primitive organisms, cnidarians are carnivorous.
Many species of Cnidaria exhibit alternation of generations and have both sexual and asexual stages of reproduction. The medusa is a free-swimming adult stage that reproduces sexually. The polyp is a benthic, sessile larval stage that reproduces asexually (figure 23.12). Sea anemones and corals are polyps. Some species lack a medusa stage in their life cycles and the polyps produce eggs and sperm. Corals secrete an external skeleton of calcium carbonate or other material. The polyp lives in a depression in the skeleton into which it can withdraw. Polyps extend from the depression when they feed. Jellyfish are free-swimming medusas; they often produce small polyp stages during their life cycle.
FIGURE 23.12. A Cnidarian Life Cycle
The life cycle of Aurelia is typical of the alternation of generations seen in most species of Cnidaria. The free-swimming adult medusa (jellyfish) reproduces sexually, and the resulting larva develops into a polyp. The polyp undergoes asexual reproduction, which produces the free-swimming medusa stage.
Members of Ctenophora (pronounced tin'-off-er-ah) are similar to cnidarians in that they are diploblastic and have radial symmetry. They have rows of cilia that allow them to swim, and many have two long, armlike appendages, which help them gather food (figure 23.13).
FIGURE 23.13. Ctenophora
Ctenophorans are marine, planktonic organisms that swim with rows of cilia. Many are bioluminescent.
23.6. CONCEPT REVIEW
15. How do sponges feed?
16. What are spicules?
17. What are the differences between a polyp and a medusa?
18. List three structural characteristics of cnidarians.