AP Biology Notes Unit 9: Structure and Function of Animals Introduction to Vertebrates A. Chordate Characteristics All chordates are bilaterally symmetrical deuterosomes that have four anatomical structures that appear at some point during the animal’s lifetime. Often these structures only appear during the embryo stage. 1. Notochord A longitudinal, flexible rod located between the gut and the nerve chord. It is composed of large, fluid‐filled cells encased in a stiff fibrous tissue. The notochord extends through most of the length of the animal as a simple skeleton. Chordates are named after this structure. In simple chordates, the notochord supports the adult. In more complex vertebrates, a jointed skeleton develops and the notochord exists as the gelatinous material of the disks between vertebrae. 2. Dorsal, Hollow Nerve Chord This develops from the neural plate ectoderm that rolls into a tube dorsal to the notochord. The dorsal, hollow nerve chord forms the central nervous system. 3. Pharyngeal Slits The lumen of the digestive tube of almost all chordates opens to the outside through slits located on the side of the pharynx. They have been modified for gas exchange, or other functions in chordates. 4. Muscular Postanal Tail Most chordates have a tail extending beyond the anus. The tail contains skeletal and muscle elements. B. Phylum Chaetognatha (Arrow Worms) There are 60 species of this arrow shaped hermaphrodite. Although the arrow worm is only 1‐10 cm in length, it is the most abundant predator of marine plankton. C. Phylum Hemichordata (Acorn Worms) Although the anterior end of the acorn worm has a probiscus, the acorn worm has pharyngeal gill slits. Water goes through the mouth, through the gills and oxygen is removed. The larval forms of acorn worms resemble larval form of echinoderms. D. Phylum Chordata (Divided into Three Subphyla) Urochordata, Cephalochordata, and Vertebrata 1. Subphylum Urochordata The most common examples of urochordates are tunicates and sea squirts. They are chordates because their tadpole‐like larvae have a notochord, a hollow dorsal nerve chord, gill slits and a postanal tail. However, they only exist in this form for a day or two. The larvae attach to a substrate and become adults. They are filter feeders, have an open circulatory system, and secrete cellulose, a rarity in animals. 2. Subphylum Cephalochordata (Lancelets) These organisms resemble fish. They have a prominent notochord, obvious pharyngeal gill slits, and a hollow dorsal nerve chord. Cephalochordates move like
fish and are filter feeders. 3. Subphylum Vertebrata All vertebrates have several traits in common: a. Backbone or vertebral column b. Cranial brain development c. Closed circulatory system with a dorsal aorta and ventral heart d. Gaseous exchange in gills and lungs e. Two pair of limbs f. One pair of image forming eyes g. Excretory system consolidated in a pair of kidneys h. Separate genders Ectothermic (poikilothermic): These organisms cannot regulate their own internal temperature. The temperature of the organism is determined by the environment. They are called ‘cold blooded’. This group includes all invertebrate phyla and vertebrate groups except birds and mammals. Endothermic (homeothermic): These organisms maintain a constant internal temperature, regardless of the environment. They are ‘warm blooded’. This group includes birds and mammals. E. Classes of Vertebrates There are eight classes of living vertebrates and one class of extinct vertebrate. 1) Class Agnatha (Jawless fish; lampreys and hagfish) 2) Class Placodermi (first fish with jaws; extinct) 3) Class Chondrichthyes (cartilagineous fish; sharks, skates, and rays) 4) Class Osteichthyes (bony fish) 5) Class Amphibia (frogs, toads, and salamanders) 6) Class Reptilia (turtles, snakes, lizards and crocodilians) 7) Class Aves (birds) 8) Class Mammalia (mammals) 1. Class Agnatha (Jawless Fish) There are about 60 modern species of jawless fish. There are fossils of some jawless fish that are as old as one half billion years old. Some agnathans suck up nutrients from sediments. Lampreys and hagfish are modern day agnathans. They have long cylindrical bodies, simple fins that are adapted for wriggling along the bottom, and feed using a rounded sucker mouth with horny spikes and a raspy tongue. Hagfish lack sucking mouths and bore into bodies of dead or dying animals. Lampreys are parasitic and attach to a body, bore into the skin and suck out blood or body fluids. The mouth is used for feeding and for the intake of water that passes over the gills. 2. Class Placodermi (Extinct Jawed Fish) Placoderms became extinct 150 million years ago. This armored group had hinged jaws, could swim better than agnathans, and were predators. The hinged jaw is an important evolutionary structure. From placodermi, chondrichthyes and osteichthyes arose. 3. Class Chondrichthyes (Sharks, Skates, and Rays)
There are about 750 species of chondrichthyes. They have a stiff caudal tail for propulsion, dorsal fin for stabilizer, pectoral and hind fins to provide lift in the water. Chondrichthyes are cartilaginous fish; the skeleton is made of cartilage. Instead of armor, they have tough skin. The skin and teeth of sharks consist of placoid scales which are made continuously. 4. Class Osteichthyes (Bony Fish) There are 36,000 species of osteichthyes. The fins are finer than previous classes of fish, and provide greater maneuverability. They have an air bladder that allows the fish to remain stationary at any depth. The body is covered with flattened scales. The skin contains numerous mucous glands which makes the fish slimy. Osteichthyes have taste buds, but their tongue is immovable. There are balance centers in their inner ear. They have a closed circulatory system with a two chambered heart. Osteichthyes are able to suck water over their gills with their mouth parts, and each gill has a food raker which keeps the food out of the gills. Most osteichthyes are oviparous. 5. Class Amphibia There are 4,000 species of modern amphibians, represented by three orders: a. Urodela Salamanders b. Anura Frogs and toads c. Apodia Worm‐like caecilians Nearly all amphibians reproduce and develop in aquatic habitats. They have moist and highly vascularized skin. The skin is the most important organ of the respiratory exchange in spite of the presence of lungs in most. They have a three chambered heart. Most frogs and toads are external fertilizers, but salamanders and caecilians are internal fertilizers. Most amphibians are oviparous with external fertilization. However, some are viviparous or ovoviparous. 6. Class Reptilia There are 7,000 species of reptiles which are represented by three important orders. a. Chelonia Turtles b. Crocodilian Crocodiles, alligators, and relatives. c. Quamata Lizards and snakes. Reptiles are conceived, live and die on land. Since they are the first fully terrestrial group of vertebrates and therefore, have specialized organs for life on land. The female reptiles retained a cloaca, but the male has developed a penis for copulation and internal fertilization. The amniote egg is porous, leathery and complete with
food and fluids (parts: yolk, allantois, chorion, embryo). Reptiles convert nitrogenous wastes to uric acid rather than ammonia. Ammonia is less toxic, but requires a large amount of water. The skin is dry with protective scales, reducing water loss. There are few mucous secreting glands. Most have a three and half or four chambered heart, with two atria and a ventricle with a partial septum. The crocodile has a complete septum and a four chambered heart. Most reptiles are carnivorous. They locate food by sight, heat detecting, olfactory and hearing. 7. Class Aves (Birds) There are 8,600 species of birds. They have a light skeleton with many hollow bones. The reptilian teeth have been replaced by a light horny beak, the neck is long and flexible, the bones of the trunk are fused together, and their breast bone is enlarged which acts as a large keel for the attachment of flight muscles. The bird tail is small made of only four vertebrae; legs are adapted for perching and grasping. Feathers evolved from reptilian scales. The feathers are strong for their weight because of their interlocking barbs. Birds have a complicated respiratory system and lack a urinary bladder; both solid waste and liquid waste are added together. The female has only one ovary to produce eggs. Birds have a four chambered heart. 8. Class Mammalia (Mammals) Mammals are furry or hairy animals that produce milk. There are three groups of mammals: monotremes (egg laying,���mono = single, treme = holed. Monotremes have retained the cloaca.), marsupials (pouched mammals, most of embryonic development occurs outside of the womb and in the pouch) and placental (development of embryo occurs in womb). Mammals use a muscular diaphragm to move air. They have a four chambered heart and lungs. Fertilization and development is usually internal (except for monotremes). Females have separate urinary and reproductive tracts. Mammals may have specialized teeth. Milk is modified sweat which provides the young with high protein, high caloric nutrients.