How do hydras ( left, Hydra ) differ from crayfish ( center , green crayfish, Barbi ) and humans ( right , Homo )? They are all multicellular heterotrophic organisms but differ according to their level of organization (hydras have only tissues but crayfishes and humans have organ systems); symmetry (hydras are radially symmetrical but crayfish and humans are bilaterally symmetrical with cephalization); and body plan (hydras have a sac body plan while crayfish and humans have a tube-within-a-tube body plan).
All animals are believed to be descended from protists; the Porifera (sponges) with the cellular level of organization may have evolved separately.
Mesoderm is the third germ layer.
Radially symmetrical animals tend to be sessile. This type of symmetry is useful because it allows these animals to reach out in all directions from one center. Bilaterally symmetrical animals tend to be active and to move forward with an anterior end.
When the first embryonic opening becomes the mouth, the coelomate is a protostome. When the first opening is the anus and the second opening becomes the mouth, the animal is a deuterostome.
In a sponge, the body wall contains two layers of cells: the outer epidermal cells and the inner collar cells. The collar cells (enlarged) have flagella that beat, moving the water through pores as indicated by the small black arrows in the figure at left. Food particles in the water are trapped by the collar cells and digested within their food vacuoles. Amoeboid cells transport nutrients from cell to cell; spicules form an internal skeleton in some sponges.
a. The life cycle of a cnidarian. Some cnidarians have both a polyp and a medusa stage; in others, one form is dominant, and in still others, one form is absent altogether. b. The anemone, which is sometimes called the flower of the sea, is a solitary polyp. c. Corals are colonial polyps residing in a calcium carbonate or proteinaceous skeleton. d. Portuguese man-of-war is a colony of modified polyps and medusae. e. True jellyfish undergo a complete life cycle; the photo shows the medusan stage.
Center: The body of Hydra is a small tubular polyp whose body contains two tissue layers. Left: Various types of cells in the body wall. Right, top: Cnidocytes are cells that contain nematocysts. Right, bottom: Hydra reproduces asexually by forming outgrowths called buds, which develop into a complete animal.
a. The micrograph of Dugesia shows that this flatworm is bilaterally symmetrical and has a head region with eyespots. b. When the pharynx is extended as shown, food is sucked up into a gastrovascular cavity that branches throughout the body. c. The excretory system with flame cells is shown in detail. d. The reproductive system has both male and female organs, and the digestive system has a single opening. e. The nervous system has a ladder-like appearance.
Tapeworm reproduction is by many repeating segments called proglottids, each a bag of eggs. Tapeworm integuments resist the hosts’ digestive juices and it absorbs its food.
Schistosomiasis, an infection of humans caused by blood flukes, is an extremely prevalent disease in Egypt – especially since the building of the Aswan High Dam. Standing water in irrigation ditches, combined with unsanitary practices, has created the conditions for widespread infection.
a. Flatworms have no body cavity, and mesodermal tissue fills the interior space. b. Roundworms have a pseudocoelom, and the body cavity is incompletely lined by mesodermal tissue. In animals that have no other skeleton, a fluid-filled coelom acts as a hydrostatic skeleton. c. Humans are true coelomates, the body is completely lined by mesodermal tissue, and mesentery holds the internal organs in place.
a. The roundworm Ascaris. b. Roundworms such as Ascaris have a pseudocoelom and a complete digestive tract with a mouth and an anus. Therefore, roundworms have a tube-within-a-tube body plan. The sexes are separate; this is a male roundworm. c. The larvae of the roundworm Trichinella penetrate striated muscle fibers, where they coil in a sheath formed from the muscle fiber.
Good hygiene, proper disposal of sewage, and cooking meat thoroughly usually protect people from parasitic roundworms.
Trochophore larvae also have a band of cilia at the midsection.
In protostomes, the first embryonic opening, called the blastopore, becomes the mouth.
a. A chiton ( Tonicella ) has a flattened foot and a shell that consists of eigfht articulating valves. b. A chambered nautilus ( Nautilus) achieves buoyancy by regulating the amount of air in chambers of its shell. c. A scallop ( Pecten ) has sensory tentacles extended between the valves. d. A nudribranch (sea slug - Flabellina ) lacks a shell, gills, and a mantle cavity. Dorsal projections function in gas exchange.
Table 30.1 (page 622) compares the features of clams a squids.
The shell and the mantle have been removed from one side of this clam. Trace the path of food from the incurrent siphon past the gills, to the mouth, the stomach, the intestine, the anus, and the excurrent siphon. Locate the three ganglia: anterior, foot, and posterior. The heart lies in the reduced coelom.
Each segment of an earthworm has its own set of longitudinal and circular muscles and its own nerve supply, so each segment or group of segments can function independently.
a. Clam worms are predaceous polychaetes that undergo cephalization. Note also the parapodia, which are use for swimming and as respiratory organs. b. Fan worms (a type of tube worm) are sessile filter feeders whose ciliated tentacles spiral in this example.
External segmentation reflects a coelom divided by septa. The brain connects to the ventral nerve chord with a lateral nerve on most segments. Nephridia in most segments remove wastes.
The top diagram shows the external anatomy of an earthworm. Note the clitellum. The bottom diagram is a cross section of an earthworm showing the internal anatomy and how the setae project through the body wall.
When earthworms mate, they are held in place by a mucus secreted by the clitellum. The worms are hermaphroditic, and when mating, sperm pass from the seminal vesicles of each to the receptor vesicles of the other.
The Health Focus (page 625) discusses the medicinal value of leeches.
Over one million species of arthropods have been described but many more may exist. Each of the five major groups of arthropods contains species that are adapted to terrestrial life. The exoskeleton contains chitin.
a. A millipede (flat-backed millipede, Sigmoria ) has only one pair of antennae, and the head is followed by a series of segments, each with two pairs of appendages. b. The hairy tarantulas of the genus Aphonopelma are dark in color and sluggish in movement. Their bite is harmless to people. c. A crab (dungeness crab, Cancer ) is a crustacean with a calcified exoskeleton, one pair of claws and four other pairs of walking legs. d. A wasp (paper wasp, Polistes ) is an insect with two pairs of wings, both used for flying, and three pairs of walking legs. e. A centipede (stone centipede, Lithobius ) has only one pair of antennae, and the head is followed by a series of segments, each with a single pair of appendages.
a. Externally, it is possible to observe the jointed appendages, including the swimmerets, the walking legs, and the claws. These appendages, plus a portion of the carapace, have been removed from the right side so the gills are visible. b. Internally, the parts of the digestive system are particularly visible. The circulatory system can also be clearly seen. Note the ventral solid nerve cord.
a. Walking sticks ( Diapheromera ) are herbivorous, with biting and chewing mouthparts. b. Bees ( Apis ) have four translucent wings and a thorax separated from the abdomen by a narrow waist. c. Flies (housefly, Musca ) have a single pair of wings and lapping mouthparts. d. Dragonflies ( Aeshna ) have two pairs of similar wings. They catch and eat other insects while flying. e. Butterflies (American copper butterfly, Lycaena ) have forewings larger than their hindwings. Their mouthparts form a long tube for siphoning up nectar from flowers.
a. Externally, the tympanum receive sound waves, and the hopping legs and the wings are for locomotion. b. Internally, the digestive system is specialized. The Malpighian tubules excrete a solid nitrogenous waste (uric acid). A seminal receptacle receives sperm from the male, which has a penis.
Chiggers are the larvae of certain mites that feed on the skin of vertebrates.
a. A scorpion (Kenyan giant scorpion, Pandinus ) has pincerlike chelicerae and pedipalps; its long abdomen ends with a stinger that contains venom. b. Most spiders are harmless, but the venom of the black widow spider ( Latrodectus ) is harmful to humans. c. In the western United States, the wood tick ( Ixodes ) carries a debilitating diseases called Rocky Mountain spotted fever. d. Arachnids breathe by means of book lungs, in which the “pages” are double sheets of thin tissue (lamellae).
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