Introduction to Vertebrates:
Chordate Characteristics: All chordates are bilaterally symmetrical deuterostomes 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.
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.
Phylum Hemichordata: acorn worms.
Although the anterior end of the acorn worm has a proboscis, 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.
Phylum Chordata: Divided into three subphyla. Urochordata
Cephalochordata Vertebrata
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.
Subphylum Cephalochordata: lancelets.
These organisms resemble fish. They have a prominent notochord,
Subphylum Vertebrata:
All vertebrates have several traits in common: 1) Backbone or vertebral column.
2) Cranial brain development.
3) Closed circulatory system with a dorsal aorta and ventral heart. 4) Gaseous exchange in gills and lungs.
5) Two pairs of limbs.
6) One pair of image forming eyes.
7) Excretory system consolidated in a pair of kidneys. 8) Separate genders.
There are two super classes and six classes of living vertebrates and one class of extinct vertebrate. 1) Super class Agnatha: jawless fish.
Cl. Cephalaspidomorphi: lampreys and Cl. Myxini: hagfish. Super class Gnathostomata:
2) Cl. Placodermi: first fish with jaws. These fish are extinct.
3) Cl. Chondrichthyes: cartilaginous fish. Examples are sharks, skates and rays. 4) Cl. Osteichthyes: bony fish.
5) Cl. Amphibia: frogs, toads, and salamanders
6) Cl. Reptilia: turtles, snakes, lizards and crocodilians. 7) Cl. Aves: birds.
8) Cl. Mammalia: mammals.
Some Definitions:
Thermoregulation: Ectothermic/Endothermic:
Ectothermic (poikilothermic): these organisms cannot regulate their own internal temperature. The environment determines the temperature of the organism. 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.
Fertilization: External—occurs outside of the female. Internal—occurs inside the female.
Reproduction:
Oviparous: Egg laying.
Viviparous: Give birth to live young.
Ovoviviparous: These have internal (soft) eggs that hatch inside the female. The young appear to have live births.
Super class Agnatha: Jawless fish.
There are about 60 modern species of jawless fish. These are Ectothermic, external fertilizers that are oviparous.
There are fossils of some jawless fish that are as old as one half billion years old. Some agnathans suck up nutrients from sediments
that are adapted for wriggling along the bottom, and feed using a rounded sucker mouth with horny spikes and a raspy tongue.
Cl. Myxini: Hagfish lack sucking mouth and bore into bodies of dead or dying animals. Cl. Cephalaspidomorphi: 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.
Super class Gnathostomata:
Class Placodermi: extinct jawed fish.
Placoderms became extinct 150 million years ago. This armored group had hinged jaws, could swim better than agnathans, and was predators. The hinged jaw is an important evolutionary structure.
From placodermi, chondrichthyes and osteichthyes arose.
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 (evolved from a bony one). Instead of armor, they have tough skin. The skin and teeth of sharks consist of placoid scales, which are made continuously.
Sharks gain buoyancy by storing a large amount of oil in the liver. The largest shark is a basking shark, a filter feeder. Sharks have a unique digestive feature called a spiral valve (their intestine). It is a spiraling flap of absorptive tissue, which increases the surface area of the intestine. Sharks swallow large chunks of food. The spiral valve slows the movement of food, allowing the enzymes time to work. The digestive system ends in a cloaca, posterior cavity that receives the ducts of the urinary, digestive and reproductive systems. The external opening is called a vent. Their digestive system (along with all verts) is like this—mouthpharynxesophagusstomachintestine (spiral valve)Cloaca (one opening for the digestive, excretory, and reproductive systems).
Sharks have keen senses. They can locate prey by smell up to 400 meters away, certain patterns of vibrations are picked up by the lateral line organ and sight. The lateral line organ, which all fish have, is composed of canals running the length of the body and contains groups of sensory cells that respond to water movements.
Sharks must keep moving to keep water moving over their gills. They lack the mouth muscles to suck water over gills. They must swim with their mouth open—must have moving water over their gills all the time. They have 5 pairs of gill slits on each side of their body.
Sharks fertilize internally. Some sharks are oviparous, ie. They lay eggs. Other sharks are ovoviviparous, ie. They lay eggs internally, the eggs hatch, and it appears that the shark is giving birth to live young. Viviparous means to give birth to live young that have developed internally but not within an egg.
There are 36,000 species of osteichthyes. They are Ectotherms, external fertilizers, and oviparous; however, some are internal fertilizers and ovoviviparous. The fins are finer that 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.
Digestive system: Mouth to Cloaca (look at shark). However, they do not have a spiral valve.
They have a closed circulatory system with a two-chambered heart.
Osteichthyes are able to suck water over their gills with their mouthparts, and each gill has a food raker, which keeps the food out of the gills. They have a bony covering over their gills called the operculum that protects the gills.
Class Amphibia: Amphibians: toads, frogs and salamanders
There are 4,800 species of modern amphibians, represented by three orders: Urodela: salamanders
Anura: frogs and toads Apodia: worm-like caecilians
These are all ectotherms. 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.
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.
They are all born in an aquatic environment and can live on land in the adult form. However, they must return to the water to reproduce.
Class Reptilia: Turtles, snakes, lizards, crocodilians, etc.
Ectothermic, internal fertilizers (see below) that are oviparous, ovoviviparous, and vivparous. There are 7,000 species of reptiles, which are represented by four important orders.
a)Testodirea: turtles
b) Crocodilia: crocodiles, alligators, and relatives. c) Squamata: lizards and snakes.
d) Sphenodontia: Tuataras
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 with the Albumin (egg whites) act as a food source to the embryo, allantois collects wastes from the embryo, chorion (soft-shell) aides in gas exchange, amnion protects the embryo along with amniotic fluid, and the 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 (must not lose water). Most have a three and a 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.
They have well developed lungs.
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 birds 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. The male has no penis—they ejaculate on their feathers and rub cloacas. They are oviparous with a hard shell.
Digestion: MouthpharynxEsophagusCropGizzard (the crop and gizzard take the place of the stomach. They both do the same thing as the stomach, but it is broken up. The crop stores and moistens the food. The gizzard grinds up the food)intestinecloaca.
Birds have a four-chambered heart.
Birds are the first homeothermic (Endothermic) class of organisms.
Class Mammalia: mammals.
Mammals are furry or hairy animals that produce milk. They are endothermic and have internal fertilization. There are three groups of mammals: monotremes (egg laying, mono = single treme = holed. Monotremes have retained the cloaca. The only oviparous mammal), marsupials (pouched mammals, most of embryonic development occurs outside of the womb and in the pouch) and placental (development of embryo occurs in womb).
Monotreme: egglayers that sweat milk and have a cloaca (spiny anteater and duck-billed platypus) Marsupials: immature young leave the reproductive tract and crawl into the pouch. There they attach to a nipple and nurse. The Opossum is the only North American marsupial. There are only 3 species of marsupials to live outside of Austrailia.
Placentals: the young develop internally, the mother provides food and nutrition through a placenta.
Respiration: Lungs
Circulation: closed with a 4 chambered heart.
Mammals are also homeothermic.
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 tract (the females have three openings while males only have two).
