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Zoology Chapter 29

Page history last edited by Shelly Turner 14 years, 4 months ago

Chapter 29:  Simple Invertebrates

 

Using your vocabulary words, make notecards/flashcards that you can use outside of class to study. 

 

Work on your Directed Reading in class every chance you get.  Remember that I will assist you on the harder questions.  This assignment is due the day we take the test on this chapter.  Use the information in this assignment as a study guide for your test.

This study guide is to help you study for your test.  It should not be the only item you use to study w hen preparing for the test.  Remember to complete your Directed Reading for every chapter along with writing your vocabulary words on notecards to help you remember them.  And always remember to listen carefully in lecture because all of the information in this study guide will be covered.

 

Vertebrates and Invertebrates

Scientists make a major distinction between vertebrates, ani­mals with a backbone or a spine, and invertebrates, animals without a backbone. Humans and other mammals are verte­brates, as are fish, amphibians, reptiles, and birds. Vertebrates are included in the phylum Chordata. Although they are the most widely recognized and familiar of all animals, vertebrates make up only about 3 percent of the more than 1 million species of animals. Vertebrates are categorized by bilateral symmetry and cephalization.

 

About 97 percent of the animal kingdom are invertebrates. Invertebrates include sponges, jellyfish, starfish, worms, mollusks, insects, and crabs. Some species of invertebrates have radial symmetry, and some bilateral. Some, but not all, species are characterized by cephalization.

 

Sponges

 

Sponges are so unlike other animals that they are sometimes put in their own subkingdom. Sponges have the simplest body organ­ization of any animal. They have no head, no mouth, and no digestive, circulatory, or nervous system. Early naturalists clas­sified sponges as plants, or as plant-animals, mainly because of their branchlike forms and their inability to move around. Sponges were not classified as animals until the mid-1800s, when scientists first began to closely study sponges and their method of feeding.

 

Most sponges live in shallow seas, though some have been found at depths of 8,500 m (27,800 ft.). A few species live in fresh water. In size, sponges range from less than 1 cm (0.4 in.) to more than 2 m (6.6 ft.). In color, they range from white and gray to brilliant shades of red, yellow, green, purple, and black. Sponges have many shapes—they may look like balls, discs, vases, goblets, branching shrubs, or small trees. The approxi­mately 5,000 species of sponges make up the phylum Porifera (puh RIHF uhr uh). Porifera means "pore bearing." Sponges have been described as looking like sacks full of holes.

 


 

 

 

Characteristics of Sponges

Once a sponge attaches itself to a rock, shell, or other sub­merged object, it does not move. Sponges feed by filtering food and nutrients out of the water. The body wall of a sponge con­sists of two layers of cells, with a jellylike layer between them. The outer layer is called the ectoderm, and the inner layer the endoderm. The body wall surrounds a cavity through which water flows. Water is drawn in through the incurrent pores in the ectoderm and leaves through a larger opening called the osculum (AHS kyuh luhm). The water carries food and dissolved oxygen. Cells that digest the food, called collar cells or choanocytes (koh AN uh syts), are located in the endoderm. Each collar cell has a flagellum.

 

The movement of the flagella sets up a current of water through filaments that make up the collar of the cells. The filaments catch and remove bacteria, unicellular algae, and other microorganisms from the water. This food is drawn into the collar cells and digested. Amoeba-like cells called amoebocytes (uh MEE buh syts) in the jellylike layer carry the nutrients to the other body cells and take away waste matter.

 

Without some kind of skeletal structure, sponges would col­lapse under their own weight. The composition of their skeletal structure is the basis of classification of sponges. Some sponges have skeletons made of spicules (SPIHK yoolz), tough interlock­ing spikes of either calcium or silicon. Other sponges have skeletons made of a flexible protein called spongin. Bath sponges, for example, have skeletons composed of spongin, which makes them firm but soft. Some sponge skeletons are a mixture of both spicules and spongin.

 


 


 


 

 

 

Sponges may grow individually or in colonies. Some colonies are so dense that it is difficult to distinguish one sponge from its neighbor.

 

Sponges have a remarkable ability to regenerate—that is, to grow new parts to replace those that are lost. A sponge not only can regenerate parts of its body, but can also regenerate the entire body from fragments. A sponge can also reform if it has been separated into single cells by being pushed through a fine silk cloth. The cells will move around, form clumps, and then larger groups. Within a few days the group of cells will reform into several new sponges. If sponges of two different species are pushed through a screen and mixed together, the fragments will regroup into sponges of the original two species.

 

Reproduction of Sponges

Sponges reproduce both sexually and asexually. Most species are hermaphrodites (huhr maf run dyts)—that is, an individual sponge can produce both eggs and sperm, though at different times. Sperm cells produced by one sponge are carried to an­other sponge by water currents. Once inside the sponge, sperm cells are captured by carrier cells, which are modified collar cells. The carrier cells take the sperm to the egg. The fertilized egg develops into a larva (lahr vuh), an immature form. Sponge larvae have flagella and swim about. They soon attach themselves to objects in the water and develop into adult sponges.

 

Fragmentation is a common method of asexual reproduction among sponges. Even a small branch that breaks off from the parent sponge can develop into a full-grown animal. Sponges also reproduce asexually by producing gemmules (jehm yoolz)— special food-filled balls of amoebocytes surrounded by protec­tive coats. Generally only freshwater sponges produce gemmules. The gemmules can survive harsh conditions, such as extreme cold or periods of dry weather. When conditions are favorable for growth, sponge cells emerge through an opening in the gemmules and grow into new sponges.

 


 

 


 

Coelenterates

Coelenterates (sih LEHN tuh rayts) are animals with saclike bod­ies and long, flexible appendages called tentacles. Most coelen­terates live in sea water. One group, the hydras, lives in fresh water. The 9,000 species of coelenterates also include jellyfish, sea anemones, and corals. Although corals are one of the sim­pler forms of animal life, they have had profound effects on the geography of continents and islands. Millions of tiny coral skeletons massed together over centuries have formed entire islands and offshore reefs. These islands and reefs are found pri­marily in the South Pacific Ocean, in the Caribbean Sea, and along the Florida coast.

 

 

Characteristics of Coelenterates

Coelenterates get their name from their coelenteron, or "hollow gut." The coelenteron is a digestive cavity with only one open­ing. Coelenterates also have special stinging cells called cnidocytes (nyd uh syts). For this reason, they are also known as cnidarians (ny dair ee uhnz).

Coelenterates generally exhibit radial symmetry. They have two body plans: vase-shaped and bell-shaped. Hydras and some other coelenterates develop only a vase-shaped body, called a polyp. Jellyfish and some other coelenterates go through a polyp stage but spend most of their lives as a bell-shaped medusa (muh doo suh).


 

 


 

Coelenterates live singly or in colonies. Individual animals within a coelenterate colony may have specialized functions. The Portuguese man-of-war is an example of a colonial coelen­terate. Some of the individual animals specialize in reproduc­tion or feeding, others in gathering food.

 

The bodies of both polyps and medusae consist of two lay­ers of cells, the endoderm and the ectoderm, separated by a jellylike substance called mesoglea (mehz uh glee uh). The mesoglea of a polyp is thin. In a medusa, however, the mesoglea often makes up the major part of the body substance.

The tentacles of most coelenterates circle the mouth of the animal, and the cnidocytes are in the tentacles. Inside each cnidocyte is a sac containing a coiled stinger, called a nematocyst (neh MAT uh sihst). When discharged, the stinger can para­lyze or lasso small prey. New cnidocytes replace those whose nematocysts have been discharged. The tentacles draw the food up to the mouth. Enzymes inside the digestive cavity break down the food, and the food particles are absorbed by the cells that line the cavity. Undigested waste products are expelled through the animal's mouth. Individual cells carry out respira­tion directly, taking in oxygen from the surrounding water by diffusion.

 

Hydras

Hydras and related animals make up the class Hydrozoa (hy druh zoh uh). Hydras are the most extensively studied coelenterates, but they are not typical of hydrozoans in some ways. For example, hydras have only the polyp form, while most hy­drozoans go through a medusa stage as well.

 

Hydras are only about 1 cm (0.4 in.) long. Most hydras are orange, brown, white, or gray in color, but some are green due to algae that grow in the cells of their endoderm. Hydras live in freshwater streams and ponds. They attach themselves to leaves and other debris in the water by means of a flattened adhesive base called the basal disc. Hydras move by floating, gliding, or somersaulting.

 


 

 

Although primitive animals, hydras show a great degree of specialization in their nematocysts. Four distinct types have been identified. One type of nematocyst anchors the tentacles when the animal moves, and another repels animals other than prey. A third holds the prey by winding around the animal, while a fourth nematocyst stings the prey, paralyzing it.

 


 

Like other coelenterates, hydras have no brain or central nervous system. A network of nerves, the nerve net, permits some coordination of responses and some simple movements. Hydras also have sensory cells that respond to chemical and mechanical stimuli. The animals have little control in their re­sponses to stimuli. If touched with a probe on one part of the body, for example, the whole body will contract.

 

When a hydra catches a shrimp or a water flea in its tenta­cles, a feeding response begins. The tentacles move the prey towards the mouth, the mouth opens in response to a chemical given off by the prey, and the prey is pushed in whole. Enzymes are released that digest the food, and the undigested remains are expelled through the mouth.

Hydras reproduce asexually by forming small buds on the outside of their bodies. These buds grow, and within two or three days they fall off and begin life as independent animals. Hydras also reproduce sexually, usually in autumn. When the water temperature drops, individual hydras begin to develop ei­ther egg-producing ovaries or sperm-producing testes. The sperm swims to the egg, which remains attached to the body of the hydra. After fertilization has occurred, the egg begins to divide and falls off the adult female. The young hydra emerges in the spring.

 

Hydras live individually and independently. Most other hydrozoans, however, live in colonies. Most hydrozoans also have both polyp and medusa forms at different times in their life cycle. Certain species, for example, spend most of their lives as colonial polyps. A single polyp multiplies by budding, forming a colony. Within the colony, the polyps become specialized. Some are feeding polyps, and some are reproductive polyps. Tiny medusae bud off the reproductive polyps. These medusae produce eggs or sperm. Thus, colonial polyps exhibit a division of function between feeding and reproductive forms. Other co­lonial coelenterates also show specialization of individuals.

 

 


 

Jellyfish

From a distance, jellyfish resemble inflated plastic bags. Ob­served more closely, jellyfish can be seen swimming by rhyth­mically contracting and relaxing their "bells." Jellyfish belong to the class Scyphozoa (sy fuh zoh uh), which means "cup animals." The tentacles of jellyfish with their stingers may reach up to 70 m (230 ft.) in length. Their central discs may range from 4 cm (1.5 in.) to a meter (3.3 ft.) in diameter, though one observer has recorded a central disc 3.6m (12 ft.) in diameter.

 

In the life cycle of a jellyfish, the medusa reproduces sexu­ally, and the polyp reproduces asexually. The male medusa re­leases sperm cells through its mouth, and the female releases eggs. The eggs lodge in pockets on the tentacles of the female and are fertilized. The eggs grow and develop into small, free-swimming larvae.

 

The larvae swim away and attach themselves to the sea floor. There they develop into a polyp stage that resembles the hydra. The jellyfish polyp grows and eventually produces buds. This development is the asexual phase of its reproductive cycle. The buds grow and eventually begin to form medusae, which, as they build up, resemble a stack of plates. These medusae bud off one by one and begin the cycle again. The polyp may repeat the process the following year.

 

 

Sea Anemones and Corals

The sea anemones (uh nehm uh neez) and corals belong to the class Anthozoa (an thuh ZOH uh). Anthozoa means "flower ani­mals." These coelenterates are often beautifully colored and have varied forms.

 

Sea anemones are marine polyps that inhabit coastal areas. A sea anemone has a basal disc like that of the hydra, by which the anemone attaches itself to rocks or other objects. Sea anem­ones are solitary. They feed on fish and crabs that swim within reach of their tentacles. Sea anemones digest their food in much the same way as hydras do.

 

Corals resemble sea anemones but have skeletons and live in colonies. Soft coral species have flexible skeletons. The stony coral has an external skeleton that is almost pure limestone. The skeletons of some corals are used to make jewelry. The skele­tons of dead corals accumulate and form the reefs often seen rising above the surface of tropical waters.

 


 

Chapter 29 Teacher Resources 

 

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