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Deuterostomes: Echinoderms and Chordates

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  • Phylum Echinodermata

    Echinoderms are coelomate, and deuterostomes.

    Echinoderms include sea stars (starfishes), sea urchins, sand dollars, sea cucumbers, and sea lilies. There are 6,000 species of echinoderms and all are marine. Although echinoderm adults have radial symmetry, they evolved from ancestors that were bilaterally symmetrical. They have free-swimming, bilateral larvae that metamorphose (change as they mature) into adults with radial symmetry.

    Figure : Bilateral symmetry can be seen in the sea star larvae (center of photograph).

    The adult body usually has five-part organization.

    They possess an internal skeleton (endoskeleton) composed of calcium carbonate plates just beneath the surface of the skin. The plates often bear spines that protrude through the skin.

    Echinoderms have numerous tube feet underneath each arm. The tube feet are connected to a system of pipes referred to as the water vascular system. Water enters the system by a sieve plate on the aboral surface. Each tube foot has a fleshy bulb or ampulla attached so that the entire structure looks like an medicine dropper or pipette. When muscles surrounding the ampulla contract, fluid inside the bulb moves down into the tube foot, extending it.

    Large digestive glands produce enzymes necessary for digestion.

    Sexes are separate and gametes are shed into the water. The gonads are large due to the necessity of releasing large numbers of gametes into the marine environment.

    Coelomic fluid circulates substances and carries amoeboid cells that clean up particulate wastes.

    Gas exchange is done with numerous tiny gills that extend from the surface of the skin.

    The nervous system consists of a central nerve ring with nerve branches extending into the arms. They do not have a brain.

    Sea stars (Starfish, Class: Asteroidea)

    Sea stars and their relatives are composed of a central disk, usually with five or a multiple of five arms (rays) extending outward.

    They feed on bivalves (clams) by pulling apart the shell and lowering their stomach into the mollusk, releasing enzymes and digesting the mollusk, then absorbing the digested material into the body of the starfish, where digestive glands in each arm continue the process of food breakdown.

    Other Classes

    Sea Urchins, Sand Dollars - Class Echinoidea
    Brittle Stars - Class Ophiuroidea
    Sea Cucumbers - Class Holothuroidea
    Sea Lilies - Class Crinoidea


    The diagram below shows evolutionary relationships among the major clades of chordates but avoids classification into subphylum and classes. For example, the close relationship between Myxini and the rest of the vertebrates can be seen even though there is uncertainty in the classification of Myxini.

    Chordates exhibit bilateral symmetry. Chordates have the following characteristics at some point in their life history:

    1. a dorsal, hollow nerve cord.
    2. a dorsal supporting rod called a notochord. This is replaced by a vertebral column in vertebrates.
    3. pharyngeal clefts (pouches). These develop into openings to the exterior (gill slits) in some chordates. Gill slits functioned as a mechanism for filter-feeding in primitive vertebrates. The gills of fish function in gas exchange.
    4. a postanal tail. In most other kinds of animals, the digestive tract extends the entire length of the animal.

    Lancelets, Subphylum: Cephalochordata

    Lancelets have all chordate characteristics and segmentation in the adult.

    Below: Lancelet X 40

    Tunicates (Sea Squirts), Subphylum: Urochordata


    The larvae of tunicates resemble the ancestral chordate.

    It has chordate characteristics and looks like a tadpole.

    The free-swimming larva develops into a sessile, filter-feeding adult.


    The adult has a thick-walled body sac and an incurrent siphon and an excurrent siphon.

    Gill slits are the only chordate feature retained by the adult form.

    In some tunicates, the adult form may have been lost. These animals retained the larval form as adults.


    Craniates are chordates with a head.

    The head contains sensory organs because it is in front of the animal as it moves through the environment.

    The evolution of a head with well-developed sensory organs and a corresponding large brain enabled these animals to be active and to feed by predation or other means that required an active animal.

    Hagfishes (Class Myxini)

    Hagfishes have a cartilaginous skull but do not have jaws. They do not have vertebrae; their notochord provides the support necessary for their muscles to produce movement.

    Vertebrates (Subphylum Vertebrata)

    The notochord of vertebrates is generally replaced by a vertebral column composed of numerous small bones called vertebrae that are joined together to form a flexible supporting structure. In most vertebrates, the vertebrae surround the spinal cord.

    The vertebral column allows the body to flex and provides attachment sites for muscles. In addition, it surrounds and protects the nerve cord.

    They exhibit extreme cephalization and possess complex sense organs (ex: eyes, ears).

    Lampreys (Class: Petromyzontida)

    Lampreys do not have jaws.

    Most species of lampreys are parasites. They attach to host fish and feed on the blood of the host.

    The larvae are filter feeders that live in freshwater streams. As they mature, they move downstream to the ocean (or lakes) and begin a parasitic life style.

    The skeleton is cartilage and the notocord persists in the adult.


    Gnathostomes are vertebrates with jaws.

    The evolution of jaws promoted the switch from filter-feeding to predation and thus promoted an active life style. Jaws evolved from the forward gill supports in fish.

    The appearance of jaws transformed the worlds ecology due to improved predation and herbivory. As a result, complex food chains evolved.

    Filter feeding (gill slits) became less important with the evolution of jaws because jaws allowed the animal to chew larger food items and to capture prey. Gills became more important in gas exchange.

    Aquatic gnathostomes have a lateral line system. This system is composed of a line of sensory organs on each side of the body. It is able to detect vibrations in the water.

    Cartilaginous Fish (Class: Chondrichthyes)

    Chondrichthyes include the sharks, and rays.

    The cartilage skeleton of sharks is partially hardened with calcium. This type of skeleton is strong and is more flexible and lighter than a bony skeleton.

    The bodies of cartilaginous fish are covered with small toothlike scales. Their teeth are larger versions of these scales.

    They do not have a swim bladder but the oil-storing capacity of their livers improves their buoyancy. The shape of the head and caudal fin also lift the animal as it swims. Sharks must swim to keep from sinking.

    Some sharks are fast-swimming predators; others are filter feeders.


    Fertilization is internal. The pelvic fin is used to transfer sperm to the female.

    Some species are oviparous- they lay eggs that hatch outside the mother's body; some are ovoviviparous- the eggs are retained within the body and young are born alive; and a few are viviparous- they receive some nourishment via a placenta that develops from the yolk sac of the egg.

    The reproductive, digestive and excretory system exit the body through a common opening called a cloaca.


    Osteichthyans have an ossified (bony) skeleton. The skeleton is hardened with calcium phosphate.

    Characteristics of Aquatic Osteichthyans (Bony Fish)

    The gills are covered by an operculum so that the gill chamber is enclosed and protected.

    A swim bladder and is used for buoyancy.

    It evolved from lungs.

    The gas content, and thus buoyancy, can be regulated by transfer to and from the blood.

    Bony fish have broad, flat scales. The skin contains mucous secreting glands that reduce friction as the fish swims through the water.

    A lateral line system detects vibrations in the water.

    Most species are oviparous.

    Bony fish are the largest group of vertebrates. Approximately 49,000 species have been identified.

    Ray-finned Fishs (Class Actinopterygii)

    The fins are supported by spinelike rays.

    In ray-finned fishes, the lungs gave rise to the swim bladder which gives the fish buoyancy.


    Lobe-finned fish have fins located on fleshy appendages.

    This group includes coelacanths (class Actinistia), Lungfishes (class Dipnoi) and tetrapods.

    Coelacanths were thought to have been extinct for 75 million years until one was captured in 1938 off the southeastern coast of Africa.

    The first vertebrate animals to develop lungs were fish. The lungs developed from a sac-like pocket of tissue that formed in the pharynx.

    Lungfishes are a group of lobe-finned fish that inhabit stagnant fresh water ponds that dry up. Their lungs allow them to gulp oxygen from the air when it has been depleted from the water. Their lobe fins enable them to walk under water.

    The ancestors of lungfish gave rise to amphibians.


    Frogs, salamanders and caecilians are amphibians.

    Amphibians evolved from lobe-finned fish.

    Amphibians are tied to the water

    Amphibians have small inefficient lungs and most species also use their skin for respiration. Because the skin is used in gas exchange, it must remain moist, so amphibians can not venture far into very dry habitats.

    Amphibians must reproduce in water because:

    • they have external fertilization and sperm require water to swim.
    • the eggs do not have an outer covering or shell and therefore are not protected against desiccation.
    • The larvae (tadpoles) are aquatic and use gills for respiration.

    In many amphibian species, the larvae (tadpoles) metamorphose into adults. The larval form is aquatic but the adult leaves the water and adopts an amphibious lifestyle.

    Some amphibian species exhibit paedomorphosis, a condition in which characteristics of the larva are retained in the adult. For example, the mudpuppy (Necturus) retains gills and remains aquatic.

    Amphibian populations have been declining for several decades. Some reasons for the declines may be habitat destruction, acid precipitation, increased levels of ultraviolet (U.V.) radiation, effects of introduced species, global warming, and pollution.


    Reptiles, Mammals

    Amniotic egg

    An amniotic egg is well-suited for terrestrial environments due to the development of the extraembryonic membranes listed below:

    amnion - surrounds and protects the embryo

    yolk sac - provides food

    chorion - involved in gas exchange along with the allantois

    allantois - stores wastes; involved in gas exchange along with the chorion

    Phylogeny of Extant Amniotes

    The diagram below shows proposed evolutionary relationships for five extant (living) groups of reptiles and three groups of mammals.

    Reptiles (Class Reptilia)

    This group includes the tuatara, lizards, snakes, turtles, crocodilians. Birds are a clade of reptiles. They are treated as a separate class in this chapter.

    They arose from amphibians during the Paleozoic but dominated during the Mesozoic.

    Adaptations for terrestrial living

    Reptiles have more efficient lungs than amphibians.

    Reptiles do not need to use their skin for gas exchange because the lungs are sufficient. They have a thick, scaly skin with keratin that makes it impermeable to water.

    Internal fertilization allows animals to mate without having to be in water.

    The eggs are encased in leathery shells to protect them from desiccation. This eliminates the need for a swimming larval stage. Reptiles have eggs with leathery shells. The eggs of birds and egg-laying mammals have calcareous shells.

    Most reptiles are ectothermic. They regulate body temperature by using heat sources in the environment and by engaging in behaviors that reduce heat loss or heat gain. For example, sunning heats the body. Remaining in an underground burrow may reduce the effect of extreme surface temperatures.


    Turtles have hard protective shells and do not have teeth.


    There are only two surviving species of tuatara. They are found on islands near New Zealand.

    Tuatara resemble lizards; they are 50 cm in length and may live to be more than 100 years old.

    Lizards, Snakes

    This is the most diverse group of reptiles.

    Members of this group have protective scales that overlap, enabling a high degree of flexability.

    Snakes do not have legs. Some species of snakes have vestigial pelvic and leg bones, reflecting their evolution from 4-legged ancestors.


    This group includes alligators and crocodiles. They are the largest living reptiles, some are more than 6.6 meters (21 feet) in length.

    Crocodilians and birds are descended from a line of reptiles (the archosaurs) that gave rise to dinosaurs.


    Dinosaurs were the dominant vertebrates for over 100 million years; their extinction may have been due to a comet or asteroid striking the earth.

    Molecular and structural evidence indicates that birds evolved from a line of dinosaurs called saurischians. Many biologists classify birds as saurischians.

    Birds (Class Aves)

    The oldest fossilized bird is Archaeopteryx. It had ancestral reptilian features such as teeth, clawed digits on the wings, and a long reptilian tail, as well as wings and feathers. Feathers are modified scales that insulate against heat loss and allow birds to be homeotherms (maintain a constant body temperature). Homeothermy allows them to be active when the environmental temperature is low.

    Birds exhibit complex behavior including parental care.

    Some anatomical features associated with flight are listed below.

    • Wings
    • Feathers
    • Hollow bones
    • They have an enlarged breastbone for attachment of flight muscles. The large breastbone can be seen in the photographs below.

    • A light, strong beak replaces the teeth and jaws seen in their reptilian ancestors.

    • One ovary - The gonads are reduced in size during the nonbreeding seasons.

    • A high metabolic rate provides the necessary energy needed for flight.

    • A double circulatory system enables high activity levels needed for flight. Birds have a double circulatory system with 2 atria and 2 ventricles. One atrium and ventricle provide a fast rate of blood flow through the lungs; the other atrium and ventricle provide a fast flow rate for the rest of the body, including the flight muscles.

    • Areas of the brain devoted to vision and motor coordination are well developed, enabling birds to have keen vision and motor coordination needed for flight. The cerebellum, which functions in motor coordination, is large in birds.

    • One-way air flow in lungs (described below)

    Respiratory System of Birds

    Birds have one-way flow of air in their lungs. As a result, the lungs receive fresh air during inhalation and again during exhalation. Advantages of one-way flow:

    • no residual volume; all old (stale) air leaves with each breath
    • The direction of blood movement in the lung blood vessels is at 90 degrees to the direction of air movement (called crosscurrent flow). Crosscurrent flow is not as efficient as countercurrent flow (180 degrees), but it is still more efficient than the flow in mammalian lungs.

    One-way flow is accomplished by the use of air sacs as illustrated below. During inspiration, the air sacs fill. During expiration, they empty.

    Mammals (Class Mammalia)

    Mammals developed during the Mesozoic from mammal-like therapsid reptiles.

    Characteristics of Mammals

    Hair insulates the body. Like birds, mammals are endothermic homeotherms and thus, are active in cold as well as warm temperatures.

    Mammary glands are modified sweat glands that produce milk. Young are born relatively helpless but are fed milk and cared for by one or both parents. Lactation promotes parent-young bonding.

    Mammals have a four-chambered heart and separate pulmonary (lung) and systemic (body) circulatory systems.

    Mammals walk more efficiently than reptiles because their legs are positioned further under their body.

    Mammals have a diversity of teeth for different kinds of foods: incisors, canines, premolars, and molars. Fish, amphibians, and reptiles have teeth that are all similar.

    Mammals have an extended period of care for their offspring.

    Three bones in the ear- the malleus, incus, and stapes- conduct sound from the tympanic membrane (eardrum) to the inner ear.

    Mammals have well-developed sense organs.

    Mammals have a large brain.

    Mammals have a high capacity for learning.


    Monotremes are found only in Australia and New Guinea.

    Unlike other mammals, monotremes lay eggs which the female incubates.

    They do not have nipples; milk is secreted to the surface of the skin.

    Only the duckbilled platypus and two species of echidnas (spiny anteaters) survive. Both of these species live in Australia.


    Marsupial young begin development in female's body. They are born very immature and crawl into the mother's marsupium (pouch) and attach to nipples. Embryonic development is completed while the young are in the marsupium.

    Today, marsupials are found mostly Australia.

    The marsupials diverged from the eutherians after Pangaea split apart. Marsupials developed on the land that included South America, Antarctica, and Australia. They underwent adaptive radiation in Australia and South America without competition from eutherians.

    A land bridge formed between South America and North America, allowing eutherians into South America. Eutherians may have been better competitors for food and other resources, driving many species of marsupials to extinction.

    Eutherians (Placental Mammals)

    Placental mammals are born relatively mature because a well-developed placenta for nutrient and waste exchange enables them to remain within the mother for an extended period of development.

    These mammals became dominant during the Cenozoic when they moved into habitats left vacant by the extinction of the dinosaurs.

    Evolution of Vertebrate Circulatory System

    Chambers of the Heart

    Vertebrate hearts contain muscular chambers called atria (sing. atrium) and ventricles. Contraction of the chamber forces blood out. Blood flows in one direction due to valves that prevent backflow.

    The atrium functions to receive blood that is returning to the heart. When it contracts, blood is pumped into the ventricle.

    The ventricle is the main pumping chamber of the heart. When it contracts, blood is pumped away from the heart to the body, lungs, or gills.

    In the diagrams that follow, arrows represent the direction of blood flow in blood vessels (arteries and veins). Blood pressure is represented by the thickness of the arrows. Thick arrows indicate high blood pressure. Blood that is rich in oxygen is represented by red arrows. Blue arrows represent blood that is low in oxygen after it has passed through the body tissues.

    Fish have a two-chambered heart with one atrium (A) and one ventricle (V).

    The gills contain many capillaries for gas exchange, so the blood pressure is low after going through the gills. Low-pressure blood from the gills then goes directly to the body, which also has a large number of capillaries. The activity level of fish is limited due to the low rate of blood flow to the body.

    Circulatory System of Amphibians

    Amphibians have a 3-chambered heart with two atria and one ventricle.

    Blood from the lungs (pulmonary flow) goes to the left atrium. Blood from the body (systemic flow) goes to the right atrium.

    Both atria empty into the ventricle where some mixing occurs.

    The advantage of this system is that there is high pressure in vessels that lead to both the lungs and body.

    Circulatory System of Some Reptiles

    Inmost reptiles except crocodilians and birds, the ventricle is partially divided. This reduces mixing of oxygenated and unoxygenated blood in the ventricle. The partial division of the ventricle is represented by a dashed line below.

    Circulatory System of Crocodilians, Birds, and Mammals

    Birds and mammals (also crocodilians) have a four-chambered heart which acts as two separate pumps. After passing through the body, blood is pumped under high pressure to the lungs. Upon returning from the lungs, it is pumped under high pressure to the body. The high rate of oxygen-rich blood flow through the body enables birds and mammals to maintain high activity levels.