Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

animal functions as a head. The “head” contains a mass of nerve cells that acts as a brain and specialized regions for sensing light, chemicals, and pressure.

 

Flatworms have three distinct layers of tissue, all composed of living cells. They have true organs and organ systems for digestion, movement, excretion, and reproduction. The digestive system consists of a muscular tube with one opening at the mouth. The excretory system consists of a network of water-collecting tubules that empty their contents into sacs leading to the exterior.

 

Movement occurs by the contraction of muscle cells that lie below the epidermis. The contractions are coordinated by signals from a nervous system. Most platyhelminthes have both testes and ovaries, and the organisms pair up to exchange sperm and eggs in fertilization.

 

Annelida

 

Members of the phylum Annelida include several classes of segmented worms, among them the worms found in mud and sand, the familiar earthworms, and the leeches. Segmented worms all display bilateral symmetry, cephalization, an open digestive system, segmentation, and a body cavity.

 

The body cavity is a true body cavity called a coelom. This is a fluid-filled space between the innermost cell layer and the outer two cell layers. In this space, the reproductive and digestive organs have evolved into complex structures with complex functions. The digestive system is cushioned in the coelom, so the activities of the digestive system take place without interacting with the inner or outer body walls.

 

Earthworms and other annelids have numerous segments, each separated from the others by internal partitions. Funnel-shaped excretory units called nephridia are located in most of the segments; they remove water and waste. Needed water is reabsorbed, and waste material passes out of the body through pores in the skin. Each segment has longitudinal and circular muscles that contract, compressing fluid to form a water-based skeleton called a hydrostatic skeleton.

 

The annelids have a digestive and circulatory system running their entire length. The circulatory system is closed, and blood is circulated by the contraction of several muscular vessels called hearts.

All annelids reproduce sexually.

 

Mollusca

 

Members of the phylum Mollusca are soft-bodied animals, such as the snail, clam, squid, oyster, and octopus. Generally, these animals live in water. Some secrete a hard shell. Each mollusk has a muscular organ called a foot that is used for gripping or creeping over surfaces. The animal has a head with a mouth; a brain or a sense organ; and groups of internal organs for circulation, excretion, respiration, and reproduction. All mollusks have a mantle, a thick fold of tissue that covers the internal organs. The space between the mantle and the body organs is called the mantle cavity.

 

Unlike the previous invertebrates presented in this chapter, mollusks have a well-developed circulatory system with a chambered heart for pumping blood. The blood passes through gills, where oxygen is obtained from water and carbon dioxide is given off. The circulatory system is said to be “open” because blood passes into open spaces at one point during its circulation.

 

Most mollusks have well-developed nervous systems with large brains and sense organs. Most of these animals have a closed digestive system with only one opening, and a true coelom. All animals of the phylum Mollusca have bilateral symmetry.

 

Three major classes make up the phylum Mollusca. The first class includes the gastropods, a group of land snails and slugs. The second class encompasses the bivalves, a group of oysters, clams, and other organisms with two shells. The third class is the cephalopods, a group of animals with numerous arms and suckers that extend from the “head foot”; octopuses and squids are in this group.

 

Arthropoda

 

The largest number of species in the animal kingdom belongs to the phylum Arthropoda. Members of this phylum, called arthropods, include such animals as spiders, ticks, centipedes, lobsters, and insects. All animals in the phylum have an external skeleton, a segmented body, and jointed appendages.

 

The external body skeleton of arthropods is called the exoskeleton. It surrounds the animal and provides support. The bodies of arthropods are often divided into distinct regions called the head, thorax, and abdomen. Mouthparts exist in the head region. Antennae, the sense organs, are located on the head also, if they are present. Respiration in the arthropods occurs through microscopic holes in the exoskeleton and body wall. Branching networks called tracheae extend from these holes to all parts of an arthropod’s body.

 

Five major classes divide the phylum Arthropoda. The first two classes include the centipedes and millipedes. These animals have multiple appendages and are able to move efficiently on land. Another class of arthropods is the crustaceans, including crabs, shrimp, lobster, and crayfish. In these animals, the exoskeleton, hardened with calcium salts, serves as a protective shell. The arachnids comprise the fourth class of arthropods, including spiders, ticks, mites, and scorpions. Arachnids have a fused head and thorax and have four pairs of legs.

 

The largest group of arthropods is the insects, including grasshoppers, butterflies, beetles, and cockroaches. Insects live in every conceivable environment on Earth and are among the most highly adapted of all animal species, with well-developed organs for various senses, including smell, touch, taste, and hearing. Insects have three pairs of jointed legs, and many have one or two pairs of wings. The body is divided into three parts: head, thorax, and abdomen. In many cases, the embryo develops into an immature form called the larva. The larva transforms into a pupa and then becomes an adult. This change is called metamorphosis.

 

Invertebrates

 

Animals are multicellular, eukaryotic organisms. They are different from plants in that they take in food and digest it into smaller components. They are heterotrophic rather than autotrophic. In addition to these characteristics, animals are able to move during some point of their life cycle. Also, the primary mode of reproduction in animals is sexual. An animal grows and changes from a single fertilized egg cell into a multicellular organism, passing through various stages of development during its life cycle.

 

Two major groups of animals exist in the world: invertebrates and vertebrates. Invertebrates are animals that have no backbones, while vertebrates are animals with backbones . Among the invertebrates are numerous phyla of animals comprising approximately 95 percent of all the animal species.

 

Porifera

 

The phylum Porifera includes a number of simple animals commonly referred to as sponges. Sponges filter and consume fine food particles through their pores. Most sponges live attached to rocks, plants, or other animals in marine environments.

 

The simplest sponges resemble vases or clusters of tubes with irregular shapes. Each sponge has a large central opening and hundreds of pores in the body wall. Cells lining the cavity remove and digest suspended food particles and water. Waste exits through the central opening.

 

Sponges are generally without symmetry, and they have a relatively simple cellular organization. The body wall contains a protective layer of flat cells on the outside, an inner layer of flagellated cells, and a gelatinous filling between the two layers. This gelatinous layer contains a group of wandering cells called amoebocytes. A sponge is supported by protein that may contain spikes of silicon or calcium compounds. This protein “scaffolding” is typical of the bath sponge.

 

The cells in the sponge act independently, and there is no evidence of tissue organization. Reproduction occurs by sexual or asexual means. It is possible that sponges evolved from protozoa living in colonies.

 

Nematoda

 

Members of the phylum Nematoda are roundworms, and many are microscopic.

 

Roundworms have many of the same characteristics as flatworms; for example, bilateral symmetry and cephalization. They also have a digestive tract open at both the mouth and the anus. This tract is suspended in a body cavity that is said to be false, so it is called a pseudocoel.

 

Many species of roundworms are free-living organisms that help consume dead plant and animal matter. Several parasitic roundworms cause human diseases, including trichinosis, hookworm disease, and elephantiasis.

 

Echinodermata

 

The phylum Echinodermata includes animals that have an internal skeleton and a water-based pressure system for locomotion. Members of this phylum include sea stars, brittle stars, sea urchins, and sea cucumbers. The embryo of echinoderms is different from that of all other invertebrates, and its structure suggests an ancestry to the phylum Chordata.

 

All echinoderms have spiny skin that helps protect them from predators. All are marine creatures, and all are radially symmetrical as adults. A ring of nerve tissue surrounds the mouth, and nerves branch off from this tissue to extend throughout the body.

 

Echinoderms have the ability to reproduce by regeneration, a process in which a relatively small piece of the animal can regenerate an entire body. All echinoderms have an internal support system called an endoskeleton and a large body cavity containing a set of canals called a water vascular system.

 

Chordata

 

The final phylum of animals is Chordata. All its members, called chordates, have bilateral symmetry, as well as a head, a body cavity, a digestive system, and body segmentation.

 

In addition, chordates have several unique structures. One such structure is the notochord. This is a stiff-yet-flexible rod of tissue extending the length of the animal that provides internal support. A second structure is a hollow nerve cord (also called a spinal cord) that extends the length of the animal just above the notochord. A third characteristic is the presence of gill slits: paired openings from the back of the organism’s mouth to the outside.

 

Not all these characteristics exist in the adult form of the chordates; some exist only in the embryonic form. However, all these characteristics exist at some time in the chordate’s life cycle. Most biologists agree that chordates evolved from the echinoderms. The special features of the chordates, such as the notochord, spinal cord, and gill slits, are adaptations to the environment.

 

The chordates are divided into two major groups: the chordates without backbones (the invertebrates) and the chordates with backbones (the vertebrates). The most primitive chordates are invertebrates. They include the tunicates and lancelets, both of which lack a backbone but have all the chordate traits. Both of these groups somewhat resemble tadpoles and are found clinging to rocks in marine environments. The vertebrate chordates are discussed in Chapter 22.

Chapter 21: Animals: Vertebrates

 Vertebrates

 

All members of the animal kingdom can be conveniently divided into invertebrates or vertebrates. Invertebrates are animals that lack a backbone, while vertebrates have a backbone. Invertebrates are found in all the animal phyla (see Chapter 21), including the phylum Chordata.

 

The phylum Chordata includes animals that have a notochord, a hollow nerve cord, and gill slits. Within this phylum, the vertebrates are classified in the subphylum Vertebrata. There are more than 40,000 living species of vertebrates. Vertebrates are the largest animals ever to inhabit Earth; the dinosaurs were vertebrates, as is the blue whale. The vertebrates are divided into several classes, encompassing fishes, amphibians, reptiles, birds, and mammals.

 

Fishes

 

Fishes are aquatic animals with a streamlined shape and a functional tail that allow them to move rapidly through water. Fishes exchange gases with their environment through gills, although a few species have lungs that supplement the gills for gas exchange.

 

One class of fishes called Agnatha is made up of species that have no jaws. Lampreys and hagfishes are species within this class. These fishes feed by sucking blood and other body fluids from their prey, usually other fishes.

 

Another class of fishes is Placodermi. The placoderms are now extinct and are recognized as the earliest known fishes with jaws. Placoderms were freshwater bottom-dwellers