ANIMALS, INVERTEBRATES
ANIMAL KINGDOM
Welcome to your Kingdom!
Animal diversity extends far beyond humans, dogs, cats, birds, and fish. There are up to 200 million living species of animals in the world today. This vast diversity encompasses a spectacular range of variation, from corals to crocodiles. First, we will consider the characteristics that all animals share as well as those that make them different and divide them into taxonomic groups.
Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers. Animals differ from both plants and fungi in their mode of nutrition. Plants are autotrophic (make their own food) eukaryotes capable of generating energy through photosynthesis. Fungi are heterotrophs (cannot make their own food) that grow on or near their food, releasing enzymes that digest food outside their bodies. Unlike plants, animals cannot make their own energy so they ingest food, either by eating other living organisms or by eating nonliving organic material. But unlike fungi, animals use enzymes to digest their food after they have ingested it.
Protists are prokaryotes (do not have a nucleus). Plants, fungi, and animals are eukaryotes (have a nucleus). Plants, fungi, and animals are multicellular, but animals do not have cell walls like plants and fungi. Cell walls function for structural support. Since animals do not have cell walls, their bodies are held together by proteins, the most abundant being collagen. Collagen is a protein that is secreted by animal cells and deposited outside of the cell boundary, between other cells. The area between cells is called the matrix. Thus, collagen deposits are called an extracellular matrix. Animals have two specialized types of cells that are not seen in any other multicellular organism: muscle cells and nerve cells. Therefore, animals are capable of movement and impulse conduction.
Protists usually reproduce by binary fission; they simply duplicate themselves asexually and split in two. Plants and fungi can often reproduce both sexually and asexually. Animals usually reproduce sexually. In most species, a small, flagellated sperm cell fertilizes a larger, non-motile egg, forming a zygote that contains genetic information from both parents. The zygote then undergoes a succession of mitotic cell divisions and develops with layers of embryonic tissues that will develop into a fully mature adult body.
Some animals (like humans) develop directly into adults through stages of maturation, but many animals also include at least one larval stage. A larva is a sexually immature form of an animal that looks different from the adult stage, usually eats different food, and may even have a completely different habitat than the adult, as in the case of the tadpole. Animal larvae eventually undergo metamorphosis to transform into the adult animal.
A clade is defined as a group of organisms consisting of a single common ancestor and all the descendants of that ancestor. Clades are further subdivided by their general appearance. A group of animal species that share the same level of organizational complexity is known as a grade. For instance, slugs and snails are in the same grade, even though snails have shells and slugs do not. Within a grade, organisms are further differentiated according to their body plan.
BODY PLANS
Animals can be categorized according to their symmetry (or lack thereof). Most sponges, for example, lack symmetry altogether. Among the animals that do have symmetrical bodies, symmetry can take different forms. Some animals exhibit radial symmetry, such as sea anemones: any imaginary slice through the central axis divides the animal into mirror images like slicing a pie. Bilateral symmetry is found in animals such as a lobster or human, which have a mirror image only on the right half and left half of the body, but the top and bottom are different and the anterior (head) and the posterior (tail) ends are different.
Many animals with a bilaterally asymmetrical body plan (such as arthropods and mammals) have sensory equipment concentrated at the anterior end, along with a central nervous system (brain) in the head. The symmetry of an animal generally fits its lifestyle. Many animals with radial symmetry are sessile (live attached to an object) or planktonic (drifting or weekly swimming, such as jellyfish). Their symmetry allows them to meet the environment equally well from all sides. In contrast, bilateral animals generally move actively from place to place. Their central nervous system enables them to coordinate complex movements involved in crawling, burrowing, flying, or swimming.
Those organisms that have bilateral symmetry are grouped into a category called the Bilateria. They can be further divided into invertebrates and vertebrates.
ANIMAL KINGDOM
A. PORIFERA (no symmetry; sponges)
B. CNIDARIA (radial symmetry; jellyfish)
C. BILATERIA (bilateral symmetry; all other animals)
1. INVERTEBRATES
a) PLATYHELMINTHES (flatworms)
b) NEMATODA (roundworms)
c) ANNALIDA (segmented worms)
d) MOLLUSCA (snails, clams, squid, octopus)
e) ARTHROPODA (spiders, insects, crabs)
f) ECHINODERMATA (starfish)
2. VERTEBRATES
INVERTEBRATES
Most of the animals alive today are invertebrates. Invertebrates account for 95% of known animal species and all but one of the animal phyla listed above. Invertebrates occupy almost every habitat on earth, from scalding water released by deep-sea hydrothermal events to the rocky, frozen ground of Antarctica.
A. PORIFERA (no symmetry; sponges)
Sponges are sessile and have a porous body and choanocytes. The term “sessile” refers to it being anchored in one spot. Animals that are “sedentary” are those that do not move much. Sponges are so sedentary that they were mistaken for plants by the ancient Greeks.
Sponges have complex tissues. They lack a digestive tract, lack body symmetry, and have no nerves or muscles.
Living in both fresh and marine (salt) water, sponges are suspension feeders: they capture food particles suspended in the water that pass through their body. Their body resembles a sac perforated with pores. Water is drawn through the pores into a central cavity, and then flows out of the sponge through a larger opening. Sponges lack true tissues, but the sponge body does contain several different cell types. Lining the interior of the central cavity are flagellated choanocytes, or collar cells. A choanocyte is a flagellated cell which sweeps water through a sponge’s body. The flagella move back and forth, generating a water current, and the collars trap food particles that the choanocytes then ingest by phagocytosis.
Most sponges are hermaphrodites (named for the Greek god Hermes and goddess Aphrodite), meaning that each individual has both male and female sexual reproductive organs. Almost all sponges exhibit their hermaphroditism in sequence, functioning first as one sex and then as the other. Gametes (either an egg or a sperm) arise from the choanocytes. The eggs stay within the sponge, but the sperm are carried out of the sponge by the water current. Cross-fertilization results from some of the sperm being drawn into neighboring individuals. Fertilization occurs within the sponge, where the zygotes develop into flagellated, swimming larva and disburse from the parent sponge. Upon settling on a suitable substrate, a larva develops into the sessile adult.
Sponges produce a variety of antibiotics and other compounds that researchers can use to fight human diseases. For example, one compound in marine sponges can kill penicillin-resistant strains of the bacterium Streptococcus. Other research is being done to find anti-cancer agents from sponges. Also, sponges are made of glass-like fibers that are one of the strongest glasses known to man. By studying the glass sponge, scientists could learn how to create unbreakable glass. It may also hold the secret to making glass at room temperature, instead of the extremely high temperatures required to do so today.
B. CNIDARIA (radial symmetry; jellyfish)
Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes. This is one of the oldest animal groups, and they have diversified into a wide range of both sessile and floating forms, including jellyfish, corals, and hydras. The basic body plan of a cnidarian is a sac with a central digestive compartment called the gastrovascular cavity. A single opening to this cavity functions as both mouth and anus.
There are two variations on this body plan: the sessile polyp and the floating medusa. Polyps are cylinder-shaped organisms that adhere to a substrate (like a rock) and extend their tentacles, waiting for prey. Examples of the polyp form include hydras and sea anemones. A medusa is a flattened, mouth-down version of the polyp. It moves freely in the water by a combination of passive drifting and contractions of its bell-shaped body. An example is a free-swimming jellyfish (called a “jellie). The tentacles of a jellie dangle from the mouth area, which points downward. Some cnidarians exist only as polyps or only as medusae; others have both a medusa stage and a polyp stage in their life cycle.
Cnidarians are carnivores that use tentacles arranged in a ring around their mouth to capture prey and to push food into their gastrovascular cavity, where digestion begins. The undigested remains are egested through the mouth/anus. The tentacles are armed with batteries of cnidocytes, unique cells that function in defense and the capture of prey. Cnidocytes contain cnidae (from the Greek word meaning nettle, or stinging thorn). A cnidae is a capsule-like organelle that delivers a chemical sting. Some cnidae have very long threads that stick to or entangle small prey that bump into its tentacles.
C. BILATERIA (bilateral symmetry; all other animals)
The development of a head where sensory structures are concentrated accompanied the evolution of bilateral symmetry.
1. INVERTEBRATES
a) PLATYHELMINTHES (flatworms)
1) Trematoda (blood flukes)
2) Cestoda (tapeworms)
b) NEMATODA (roundworms)
c) ANNALIDA (segmented worms)
1) Oligochaetes (earthworms)
2) Hirudinea (leeches)
d) MOLLUSCA
1) Gastropods (snails and slugs)
2) Bivalves (clams, oysters, mussels, and scallops)
3) Cephalopods (octopuses and squid)
e) ARTHROPODA
1) Cheliceriforms (spiders)
2) Myriapods (millipedes and centipedes)
3) Insects
4) Crustaceans (crabs)
f) ECHINODERMS (starfish)
PLATYHELMINTHES (flatworms)
Flatworms have no radial symmetry; they live in marine, fresh water, and damp earth habitats. In addition to many free living forms, flatworms include many parasitic species, such as flukes and tapeworms. Flatworms are named because their bodies are thin; the smallest are nearly microscopic, while some tapeworms can be over 20 m long. Flatworms have no anus, but have a highly branched gastrovascular cavity with only one opening. Although there are four classes of flatworms, we will only discuss the two that are parasites: Trematoda and Cestoda.
1) Trematodes (blood flukes) live as parasites in or on other animals. Many have suckers for attaching to internal organs or to the outer surfaces of the host. Reproductive organs occupy nearly the entire interior of these worms. Trematodes parasitize a wide range of hosts. Many require an intermediate host in which larvae develop before infecting the final host where the adult worms live. For example, Trematodes that parasitize humans spend part of their lives in snail hosts. 200 million people around the world are infected with blood flukes. Individual flukes can survive in humans for more than 40 years.
2) Cestoda (tapeworms) are parasites that live inside vertebrates, including humans. The anterior end, or scolex (head), has suckers or hooks that lock the worm to the intestinal lining of the host. Tapeworms have no digestive tract of any sort, and no gastrovascular cavity; they absorb nutrients directly from the host intestine. The length of the tapeworm is a long ribbon of units called proglottids, which are sacs of sex organs. Mature proglottids, loaded with thousands of eggs, are released from the tail end of a mature tapeworm and leave the host body in feces. If the feces contaminates food or water ingested by other animals such as pigs or cattle, the tapeworm eggs develop into larva that form cysts in the muscles of these animals. Humans acquire these larvae by eating undercooked meat, and the worms develop into mature adults within the human. Large tapeworms can block the intestines and rob enough nutrients from the human host to cause nutritional deficiencies.
NEMATODA (roundworms)
Roundworms are among the most widespread of all animals; they are found in most aquatic habitats, in the soil, in moist tissues of plants, and in body fluids and tissues of animals. They do not have a segmented body. Their size ranges from 1 mm to more than 1 m in length, tapering to a fine tip at the posterior end. The body is covered by a tough coat called a cuticle; as the worm grows it periodically sheds the old cuticle and secretes a new, larger one.
Nematodes have a digestive canal, although they lack a circulatory system. Nutrients are transported throughout the body by way of a fluid. Nematodes have muscles that produce a thrashing motion when they contract.
Nematode reproduction is sexual; the sexes are separate and females are larger than males. A female may deposit 100,000 fertilized eggs per day. There are 25,000 known species, most of which are not parasites and live in moist soil and decomposing organic matter on the bottoms of the lakes and oceans. They play an important role in decomposition and nutrient cycling.
Many nematodes are important agricultural pests that attack the roots of plants. Some nematodes parasitize animals. Humans host at least 50 nematodes species, including pinworms and hookworms. One notorious nematode is Trichenella, the worm that causes trichinosis. Humans acquired this nematode by eating undercooked infected pork with juvenile worms and encyst in the muscle tissue. Within the human intestine, the juveniles develop into adults, burrow through the body and travel to other organs, including muscles, where they encyst.
ANNALIDA (segmented worms)
Annalida means “little rings”. They live in the sea, fresh water, and damps soil. They range in length from 1 mm to 3 m, the lakes of a giant Australian earthworm. They are divided into three classes, but we will discuss two: Oligochaetes (earthworms) and Hirudinea (leeches).
1) Oligochaetes (earthworms)
Oligochaetes (“Oligo” means “few” and “chaetes” means “hairs”) are named for their sparse bristles. Earthworms eat their way through the soil, extracting nutrients as the soil passes through the digestive canal. Undigested material, mixed with mucus secreted into the canal, is excreted through the anus. Earthworms till the earth, adding to their feces to improve the texture of the soil. Earthworms are hermaphrodites, but they cross-fertilize. Two earthworms mate by aligning themselves in such a way that they exchanged sperm, and then they separate. The received sperm are stored temporarily while an organ secretes a mucus cocoon. The cocoon slides along the worm, picking up the eggs and then the stored sperm. The cocoon then slips off the worm’s head and remains in the soil while the embryos develop. Some earthworms can also reproduce a sexually by fragmentation followed by regeneration.