Introduction to Invertebrates PDF

Arthropoda, Mollusca &Echinodermata FOR SECOND LEVEL ZOOLOGY/CHEMISTRY STUDENTS (ZOOL 207) BY Prof. ASHRAF AHMED MONTASSER Prof. Dr. of Invertebrates Zoology Department 1  Phylum...

Arthropoda, Mollusca &Echinodermata FOR SECOND LEVEL ZOOLOGY/CHEMISTRY STUDENTS (ZOOL 207) BY Prof. ASHRAF AHMED MONTASSER Prof. Dr. of Invertebrates Zoology Department 1  Phylum: Arthropoda ***** Arthropds have been successful in almost all habitats on the earth. Some ancient arthropods were the first animals to live most of their lives in terrestrial environments. Members of the subphylum Trilobitomorpha are extinct arthropods that were a dominant life-form in the oceans between 345 and 600 million years ago. Zoologists have described about one million species of arthropods, and recent studies estimate that 30 to 50 million species may yet be undescribed. General characters: 1. Metamerism modified by the specialization of body regions for specific functions. 2. Chitinous exoskeleton that provides support and protection. 3. Paired, jointed appendages. 4. Growth accompanied by ecdysis or molting. 5. Ventral nervous system. 6. Coelom reduced to cavities surrounding gonads and sometimes excretory organs. 7. Open circulatory system in which blood is released into tissue spaces (hemocoel). 8. Complete digestive tract. 9. Metamorphosis often present; reduces competition between immature and adult stages. 2 Metamerism and Tagmatization: Three aspects of arthropod biology have contributed to their success. One of these is metamerism. Metamerism of arthropods is most evident externally because the arthropod body is often composed of a series of similar segments, each bearing a pair of appendages. Internally, however, septa do not divide the body cavity of an arthropod, and most organ systems are not metamerically arranged. Metamerism permits the specialization of regions of the body for specific functions. This regional specialization is called tagmatization. In arthropods, body regions, called tagmata (sing. tagma), are specialized for feeding and sensory perception (head), locomotion (thorax), and visceral organs (abdomen). 3 The Exoskeleton: An external, jointed skeleton, called an exoskeleton or cuticle, encloses arthropods. The exoskeleton is often cited as the major reason for arthropod success. It provides structural support, protection, impermeable surfaces for the prevention of water loss, and a system of levers for muscle attachment and movement. The exoskeleton has two layers. The epicuticle is the outermost layer. Made of a waxy lipoprotein, it is impermeable to water and a barrier of microorganisms and pesticides. The bulk of the exoskeleton is below the epicuticle and is called the procuticle. The procuticle is composed of chitin, a tough, leathery polysaccharide, and several kinds of proteins. The procuticle hardens through a process called sclerotization and sometimes by impregnation with calcium carbonate. The growth of an arthropod would be virtually impossible unless the exoskeleton was periodically shed, such as in the molting process called ecdysis. Arthropod Exoskeleton (After Miller and Harley, 2002) 4  Arthropopds periodically undergo ecdysis, or molting, the shedding of the outer cuticular layer. When they outgrow their exoskeleton, they form a new one underneath. This process is controlled by hormones. When the new exoskeleton is complete, it becomes separated from the old one by a fluid (gel). This fluid dissolves old exoskeleton that has some cracks. The new exoskeleton appeared pale, still somewhat soft and subsequently hardens. At this stage arthropods, often hide under stones, leaves, or branches. 5 Metamorphosis: A third characteristic that has contributed to arthropod success is a reduction of competition between adults and immature stages because of metamorphosis. Metamorphosis is a change in body form and physiology as an immature stage, usually called a larva, becomes an adult. The evolution of arthropods has resulted in an increasing divergence of body forms, behaviors, and habitats between immature and adult stages. Adult crabs, for example, usually scavenge the live preys or decaying organic matter on sandy bottoms of marine habitats. Similarly, the caterpillar (larva of butterfly or moth) that feed on leafy vegetation eventually develops into a nectar-feeding adult. Having different adult and immature stages means that the stages do not compete with each other for food or living space. In some arthropod and other animals groups, larvae also serve as the dispersal stage. 6 Classification of the phylum Arthropoda: Phylum: Arthropoda Subphylum: Trilobitomorpha Subphylum: Chelicerata Class: Merostomata Class: Arachnida Class: Pycnogonida Subphylum: Crustacea Class: Branchiopoda Class: Malacostraca Class: Copepoda Class: Cirripedia Subphylum: Uniramia Class: Chilopoda Class: Diplopoda Class: Hexapoda 7 Subphylum: Trilobitomorpha Members of Trilobitomorpha were a dominant form of life in the oceans from the Cambrian period (600 million years ago) to the Carboniferous period (345 million years ago). They crawled along the substrate feeding on annelids, mollusks, and decaying organic matter. The trilobite body was oval, flattened and divided into three longitudinal sections (thus the subphylum name). Body was also divided into 3 regions. The head carried a pair of antennae and eyes. The trunk or thorax carried appendages for swimming or walking. A series of posterior segments formed the pygidium or tail. Trilobite appendages consisted of two lobes or rami, and are called biramous appendages. The inner lobe was a walking leg and the outer lobe may have been used in digging or swimming or as gills in gas exchange. 8 Subphylum: Chelicerata Chelicerata (Gr. chele, claw + ata, plural suffix) includes familiar animals such as spiders, mites and ticks and less familiar animals as horseshoe crabs and sea spiders. These animals have two tagmata. 1) The prosoma or cephalothorax is a sensory, feeding, and locomotor region. It bears eyes, but unlike other arthropods, never has antennae. Prosoma carried one pair of chelicerae for feeding, pair of sensory pedipalps for feeding, locomotion, or reproduction and 4 pairs of walking legs. 2) Opisthosoma contains digestive, reproductive, excretory and respiratory organs. Class: Merostomata Merostomata are divided into two subclasses. The Xiphosura are the horseshoe crabs and the Eurypterida are the giant water scorpions. Subclass: Xiphosura. Only four species of horseshoe crabs are living today, ex. Limulus sp. They are scavengers for annelids, small mollusks and other invertebrates. Their body form has remained virtually unchanged for over 200 million years and was cited as an example of stabilizing selection. A hard, horseshoe-shaped carapace covers the prosoma. The chelicerae, pedipalps and first three pairs of walking legs are chelate and are used for walking and food handling. The last pair of appendages has leaflike plates at their tips and are used for locomotion and digging. The opithosoma of Limulus sp. includes a long, unsegmented telson. The first pair of opisthosomal appendages cover genital pores and are called genital opercula. The remaining five pairs of appendages are book gills. The name is derived from the resemblance of these platelike 9 gills to the pages of a closed book. Gases are exchanged between the blood and water as blood circulates through the book gills. Horseshoe crabs have an open circulatory system, as do all arthropods. Horseshoe crabs are dioecious. During reproductive periods, males and females meet in intertidal areas. The male mounts the female and grasps her with his pedipalps. The female digs shallow depressions in the sand, and as she sheds eggs into the depressions, the male fertilizes them. Fertilized eggs are covered with sand and develop independently. Limulus sp. (ventral view) (After Miller and Harley, 2002) 10 Subclass: Eurypterida. They are extinct, having lived from the Cambrian period (600 million years ago) to the Permian period (280 million years ago). They were probably inhabitants of rivers and lagoons, after a marine origin. It is believed that they were ancestors of the arachnids. Eurypterus sp. attained a large size reaching 9 feet that represent the largest known arthropod. It had a scorpion-like appearance with dorso-ventrally compressed body and covered with chitinous exoskeleton. The prosoma was formed of 6 fused segments covered by a carapace which carried 2 central simple eyes and 2 marginal compound eyes. Six pairs of legs surrounded the mouth, the first was chelate and the last was broad powerful paddle like used for swimming and burying in mud. The name Eurypterida (Gr., Eurys, broad + pteron, wing) refers to this wing like pair of legs. The opisthosoma composed of 12 segments. The anterior 6 formed the mesosoma which was broad and carried the genital operculum and 5 pairs of leaf-like respiratory appendages. The posterior 6 segments formed the metasoma which was narrow, long, limbless and terminated with long telson. 11 Eurypterus sp. (After Miller and Harley, 2002) 12  Class: Arachnida The majority of spiders, mites, ticks, scorpions and related forms are either harmless or very beneficial to humans. The earliest fossils of aquatic scorpions date from the Silurian period (405-425 million years ago), fossils of terrestrial scorpions date from the Devonian period (350- 400 million year ago), and fossils of all other arachnid groups are present by the Carboniferous period (280-345 million years ago). Form and function: Digestive system and digestion: Most arachnids are carnivores. They hold small arthropods with their chelicerae while enzymes from the gut tract pour over the prey. Partially digested food is then taken into the mouth. Others inject enzymes into prey through hollow chelicerae (e.g. spiders) and suck partially digested animal tissue. The gut tract of arachnids is divided into three regions. The anterior portion is the foregut, and the posterior portion is the hindgut. Both develop as infoldings of the body wall and are lined with cuticle. A portion of the foregut is frequently modified into a pumping pharynx, and the hindgut is frequently a site of water reabsorption. The midgut between the foregut and hindgut is noncuticular and lined with secretory and absorptive cells. Lateral diverticula increase the area available for absorption and storage. Excretory system and excretion: Arachnids use coxal glands and/or Malpighian tubules for excreting nitrogenous wastes. Coxal glands are paired, thin walled, spherical sacs bathed in the blood of body sinuses. Nitrogenous wastes are absorbed across the wall of the sacs, transported in a long, convoluted 13 tubule, and excreted through excretory pores at the base of the posterior appendages. Arachnids that are adapted to dry environments possess blind- ending diverticula of the gut tract that arise at the juncture of the midgut and hindgut. These tubules, called Malpighian tubules, absorb waste materials from the blood and empty them into the gut tract. Excretory wastes are then eliminated with digestive wastes. The major excretory product of arachnids is uric acid. Uric acid excretion is advantageous for terrestrial animals because uric acid is excreted as a semisolid with little water loss. Respiratory system and respiration: Gas exchange also occurs with minimal water loss because arachnids have few exposed respiratory surfaces. Some arachnids possess structures, called book lungs that are assumed to be modifications of the book gills in the Merostomata. Book lungs are paired invaginations of the ventral body wall that fold into a series of leaflike lamellae. Air enters the book lung through a slitlike opening and circulates between lamellae. Repiratory gases diffuse between the blood moving among the lamellae and the air in the lung chamber. Other arachnids possess a series of branched, chitin-lined tubules that deliver air directly to body tissues. These tubule systems, called tracheae (sing., trachea) open to the outside through openings called spiracles along the ventral or lateral aspects of the abdomen. 14  Arachnid Book Lung: Air and blood moving on opposite sides of a lamella of the lung exchange respiratory gases by diffusion. (After Miller and Harley, 2002) Circulatory system and circulation: The circulatory system of arachnids, like that of most arthropods, is an open system in which a dorsal contractile vessel (usually called the dorsal aorta or “heart”) pumps blood into tissue spaces. In arthropods, the coelom is reduced to cavities surroundings the gonads and sometimes the coxal glands. Large tissue spaces, or sinuses, are derived from the blastocoel and are called the hemocoel. Blood bathes the tissues and the returns to the dorsal aorta through openings in the aorta called ostia. Arachnid blood contains the dissolved respiratory pigment hemocyanin and has amoeboid cells that aid in clotting and body defenses. Nervous system and sensory structures: The nervous system of all arthropods is ventral. With the exception of scorpions, the nervous system of arachnids is centralized by fusion of ganglia. 15  The body of an arachnid has a variety of sensory structures. Most mechanoreceptors and chemoreceptors are modifications of the exoskeleton, such as projections, pores, and slits, together with sensory and accessory cells. Collectively, these receptors are called sensilla. For example, setae are hairlike, cuticular modifications that may be set into membranous sockets. Displacements of a seta initiates a nerve impulse in an associated nerve cell. Vibration receptors are very important to some arachnids. Spiders that use webs to capture prey, for example, determine both the side of the insect and its position on the web by the vibrations the insect makes while try to free itself. The chemical sense of arachnids is comparable to taste and smell in vertebrates. Small pores in the exoskeleton are frequently associated with peglike, or other modifications of the exoskeleton, and they allow chemicals to stimulate nerve cells. Arachnids possess one or more pairs of eyes, which they use primarily for detecting movement and changes in light intensity. The eyes of some hunting spiders probably form images. Arthropod Seta and Eye: a) A seta is a hairlike modification of the cuticle set in a membranous socket. Displacement of the seta initiates a nerve impulse in a receptor cell (sensillum) associated with the base of the seta. The lens of this spider eye is a thickened, transparent modification of the cuticle. Below the lens and hypodermis are light-sensitive sensillae with pigments that convert light energy into nerve impulses. 16 Reproductive system: Arachnids are dioecious (sexes separate). Paired genital openings are on the ventral side of the second abdominall segment. Sperm transfer is usually indirect. The male often packages sperm in a spermatophore, which is then transferred to the female. In some taxa (e.g. spiders), copulation occurs, and sperm is transferred via a modified pedipalp of the male. Development is direct, and the young hatch from eggs as miniature adults. Order: Scorpionida: Scorpions are common from tropical to warm temperate climate. They are nocturnal, hiding during most daylight hours under stones. Only a few scorpions have venom that is highly toxic to humans, e.g. species of Centruroides have been responsible for human deaths. Other scorpions cause stings similar to wasp stings. 17  External anatomy of a scorpion (After Miller and Harley, 2002) Scorpions have small chelicerae that project anteriorly from the front of the carapace. A pair of enlarged, chelate pedipalps is posterior to the chelicerae, and followed by 4 pairs of walking legs. The opisthosoma is divided. An anterior preabdomen (mesosoma) contains the slit-like openings to book lungs, comb-like tactile and chemical receptors called pectins and genital openings. The postabdomen (commonly called the tail) is narrower than the preabdomen and is curved dorsally and anteriorly over the body when stimulated. At the tip of the postabdomen is a sting. The sting has a bulbular base that contains venom-producing glands and a hollow, sharp, barbed point. Most arthropods are oviparous; females lay eggs that develop outside the body. Many scorpions are ovoviviparous; development is internal, although large, yolky eggs provide all the nourishment for development. Some scorpions, however, are viviparous, meaning that the mother provides nutrients to nourish the embryos. Development requires 18 up to 1.5 years and 20-40 youngs are brooded. After birth, the young crawl onto the mother's back, where they remain for up to a month. Order: Araneae: With about 34000 species, the order Araneae is the largest group of arachnids. The prosoma of spiders bears chelicerae with poison glands and fangs. Pedipalps are leg-like and in males are modified for sperm transfer. The dorsal, anterior margin of the carapace usually has six to eight eyes. A selender, waist-like pedicel attaches the prosoma to the opithosoma. The latter is swollen or elongate and contains openings to the reproductive tract, book lungs and tracheae. It also has six to eight conical projections, called spinnerets that are associated with silk glands. The protein that forms silk is emitted as a liquid, but hardens as it is drawn out. Silk webs are useful to capture the prey, to lay a safety line that fasten to the substrate to prevent falling, and to wrap eggs into a case for development. External Structure of a Spider (After Miller and Harley, 2002) 19  Most spiders feed on insects and other arthropods that they hunt or capture in webs. The venom of most spiders is harmless to humans, except some species of Lactrodectus or Loxosceles which have toxic venom to humans. Order: Acarina: Acarina includes mites and ticks, which are ectoparasites on human and domestic animals. Of all arachnids, acarines have had the greatest impact on human health and welfare. Mites are 1mm or less in length. The prosoma and opisthosoma are fused and covered by a single carapace. An anterior projection called the capitulum carries mouthparts. Chelicerae and pedipalps are variously modified for piercing, biting, anchoring, and sucking and adults have four pairs of walking legs. Free-living mites may be herbivores or scavengers. Parasitic mites usually do not permanently attach to their hosts, but feed for a few hours or days and then drop to the ground. A few mites are permanent ectoparasites. Sarcoptes scabei is the human itch mite. It tunnels in the epidermis of human skin, where females lay about 20 eggs each day. Secretions of the mites irritate the skin, and infections are acquired by contact with an infected individual. 20  Sarcoptes scabei Ixodes scapularis Ticks are ectoparasites, during their entire life history. They may be up to 3 cm in length, but are otherwise similar to mites. Hooked mouthparts are used to attach to their hosts and to feed on blood. The female ticks, whose bodies are less sclerotized than those of males, expand when engorged with blood. Copulation occurs on the host, and after feeding, females drop to the ground to lay eggs. Eggs hatch into six- legged larvae which feed on host blood and drop to the ground for each molt. Some ticks transmit diseases to humans and domestic animals, e.g. Dermacentor andersoni transmits the bacteria that cause Rocky Mountain spotted fever and Ixodes scapularis transmits the bacteria that cause Lyme disease. 21 Class: Pycnogonida Members of the class Pycnogonida are the sea spiders. All are marine and are most common in cold waters. Pycnogonids live on the ocean floor and frequently feed on cnidarian polyps and ectoprocts. Some sea spiders feed by sucking prey tissues through a proboscis. Others tear the prey with their chelicerae. Pycnogonida are dioecious. Gonads are U-shaped and branches of the gonads extend into each leg. Gonopores are on one of the pairs of legs. As the female releases eggs, the male fertilizes them, and the fertilized eggs are cemented into spherical masses and attached to a pair of elongate appendages of the male, called ovigers, where they are brooded until hatching. Pycnogonum littorale 22