Chapter 30 DEUTEROSTOMES

Echinoderms and chordates are...

  1. Deuterostomes.
  1. Radial embryonic cleavage pattern.
  1. Indeterminate cleavage: fate of cells is fixed later in development than in protostomes.
  1. Mesoderm develops from a pair of pouches that pocket out of the primitive gut: enterocoely.
  1. Cavity in mesodermal pockets becomes the coelom

Biologists include hemichordates and chaetognaths as deuterostomes.

The larva of echinoderms and hemichordates is very similar.

  • Bilateral
  • Ring of cilia around the mouth.

Himichordates and chordates have

  • Pharyngeal pouches.
  • Dorsal nerve cord.

Other phyla included sometimes among the deuterostomes are the lophophorids (Phoronida), bryozoans (Ectoprocta) and brachiopods (Brachiopoda).

Chordates have about twice the number of genes found in invertebrates.

A mutation the Cambrian (590-505 m.y.a.) could have resulted in a doubling of chromosome number.

Further mutations on genes the play a role in embryonic development could have resulted in new body designs including the vertebrate head.

ECHINODERMS

CHARACTERISTICS

  1. Coelom well developed, filled with fluid that transports materials.
  1. Larva bilaterally symmetrical; adult with radial symmetry and usually pentamerous; complex metamorphosis.
  1. Head absent; mouth facing downward.
  1. Endoskeleton of made of CaCO3 plates and bearing spines; covered with a thin ciliated epidermis.
  1. Water vascular system consisting of canals filled with seawater through an opening called the madreporite; it is derived from the coelom.
  1. Free moving animals. Locomotion by tube feet connected to the water vascular system, or by moving the spines or arms.
  1. Gas exchange through a variety of structures, e.g. gills, respiratory trees.
  1. Excretory system absent; excretion by diffusion.
  1. Digestive system complete.

10. Circulatory system very reduced and function uncertain. It may or may not be homologous to the

circulatory system of other phyla.

11. Simple nervous system; brain absent; nerve ring around the mouth with radiating nerves.

12. Reproduction sexual; sexes separate; external fertilization; a few species hermaphroditic.

Echinoderms are carnivorous, bottom dwellers of the sea. They are found at all depths. About 7,000 extant and 13,000 extinct species make the phylum.

Class Crinoidea - Body formed by a disc with a leathery skin containing calcareous plates; mouth turned upward. Five flexible arms branch form many more arms and lateral pinnules giving a feathery appearance; tube feet on the arms, coated with mucus to trap plankton. Suspension feeders.

Class Asteroidea - Sea stars typically have five arms (though they may have more) which merge gradually with the central disc. Skin gills carry on gas exchange; tiny pincer-like pedicellariae keep the skin free of debris. On the aboral surface can be found the madreporite, a disc sealing the water vascular system and used to equalize the pressure.

Class Ophiuroidea - Pentamerous (have five arms); arms slender and sharply set off from the central disc; lack pedicellariae; tube feet have no suckers, and not used in locomotion; tube feet may have a sensory function to detect food; ophiuroids use their entire arms for movement. The madreporite is located on the oral surface; anus is absent and undigested food is expelled from the mouth. E.g. Brittle stars

Class Echinoidea - Echinoids lacks arms and have a compact body encased in a flat shell called test. Their pentaradial structure is evident in the arrangement of the ambulacral areas; tube feet and spines used in locomotion. Examples: Sea urchins, sand dollars

Class Holothuroidea - Holothurians are elongated; their body is flexible and soft. They have become secondarily bilateral; their tube feet are well developed along only one ambulacral groove; endoskeleton is reduced to microscopic plates. Practice evisceration as a means of defense. Example: Sea cucumbers.

Class Concentricycloidea - These are small (less than 1 cm in diameter) disk-shaped echinoderms have a concentric vascular rings. Just discovered in 1986, attached to and in crevices in wood. At >1000m off New Zealand and later at about 2000m in the Bahamas. Like echinoids and sea cucumbers but wit no arms. They have a big coelom, with a system with tube feet that have ampullae, and have a calcareous endoskeleton of ossicles. Tube feet in just one row in a circular arrangement along the periphery of the animal. One species has no gut...absorbs nutrients presumably. In the other species, there is a mouth and stomach and no anus. Where they fit is not yet clear.

They live in submerged wood rich in bacteria. Recent cladistic studies have been made using nucleotide sequence in an attempt to understand the relationship among different echinoderm classes.

In these cladistic studies "Xyloplax is consistently placed among asteroids. This result is stable over a wide range of analysis parameters and demonstrates that Xyloplax is a recent asteroid rather than a relict stem lineage of echinoderm. Hence, the rank of Class is an inappropriate representation of the evolutionary history of the concentricycloids."

PHYLUM CHORDATA

Chordates are coelomate animals with bilateral symmetry, segmented body, with a tube-within-a-tube body plan and three well-developed germ layers, an endoskeleton and closed circulatory system. About 47,000 species.

Chordates share with other phyla the following characteristics: coelomates with bilateral symmetry, a tube-within-a-tube body plan, endoskeleton, and a close circulatory system with a ventral heart.

DISTINGUISHING CHARACTERISTICS

  1. Notochord.
  • Cartilaginous rod running underneath and supporting the nerve cord.
  • Replaced by vertebrae in the adults of many groups.
  1. Dorsal nerve cord.
  • Single, hollow, dorsal, above the notochord.
  1. Pharyngeal slits.
  • Present in the embryo and adult of some species.
  1. Post-anal tail.
  • Prominent in embryos of all groups but not in all adults.

SYSTEMATICS

PHYLUM CHORDATASubphylum Urochordata

Subphylum Cephalochordata

Subphylum Vertebrata

1. UROCHORDATES

  • Larval stage is chordate with gills, notochord and dorsal nerve cord.
  • These structures are lost in the adult stage except for the gill slits, which are present in the adult.

2. CEPHALOCHORDATES.

Share with the Vertebrata the following characteristics:

  • Notochord, gill slits, dorsal nerve cord, metameric muscles, posterior direction of blood flow in the dorsal vessel and anterior blood flow in the ventral vessel, thyroid, homologous homeobox gene clusters. Larva similar to the Agnatha.

- Notochord extends to the anterior end (cephalochordates!) and does not end at a brain.

  • Adults are suspension feeders burrowing in the sea bottom.
  • Example: Branchiostoma (=Amphioxus), lancelet

3. VERTEBRATES

  1. Vertebral column present; it replaces the notochord in most species.
  1. Pronounce cephalization: well developed brain.
  1. Cranium encloses and protects the brain.
  1. Two pairs of appendages.
  1. Muscles attached to the endoskeleton for movement.

CLASS AGNATHA Jawless fish

  • Long tubular body similar to an eel.
  • Lack jaws.
  • Cartilaginous skeleton.
  • Notochord persist throughout life.
  • Dorsal and ventral fins but lack paired fins.
  • Heart with one atrium and one ventricle.
  • Dorsal nerve cord with differentiated brain.
  • Digestive systems without stomach.
  • Sense organs of taste, smell and hearing.
  • External fertilization.

About 70 species found in both freshwater and marine environment e.g. lampreys and hagfishes.

Their fossil record goes back to the Ordovician, 450-500 m.y.a.

During the Silurian (440-408 m.y.a.) and Devonian (408-380 m.y.a.) fishes evolved jaws and paired fins.

Acanthodians and placoderms were jawed and armored.

CLASS CHONDRICHTYES Cartilaginous fishes

  • Skin with placoid scales.
  • Endoskeleton entirely cartilaginous.
  • Notochord replaced by vertebrae.
  • Two pairs of fins.
  • Two-chambered heart.
  • 5 to 7 pairs of gills with separate and exposed gill slits.
  • No swim bladder or lung.
  • Internal fertilization and separate sexes.
  • Oviparous, ovoviviparous; viviparous.

CLASS OSTEICHTHYES Bony fishes

  • Skin usually with overlapping dermal scales, some fish without scales.
  • Skeleton with many vertebrae replace the notochord.
  • Median and paired fins supported by fin rays of cartilage or bone.
  • Jawed mouth terminal; most have many teeth.
  • Gills supported by bony gill arches and covered by a common bony flap, the operculum.
  • Two-chambered heart.
  • Swim bladder usually present.
  • Sexes separate and fertilization is usually external.
  • Development oviparous.

Bony skeleton provides excellent support and stores calcium.

Descendants of the bony fishes moved onto land during the Devonian (408-380 m.y.a.).

Lobed-finned fishes are very similar to those that moved onto land.

There are three extant species of lung fishes and one coelacanth.

CLASS AMPHIBIA

  • Aquatic larva undergoes metamorphosis into a terrestrial adult in many species.
  • Skin smooth and moist with many glands, some of which may be poison glands; no scales; involved in gas exchange.
  • Respiration by lungs, skin and gills, either separately or in combination; external gills in the larval form and may persist throughout life in some.
  • Three-chambered heart: two atria and one ventricle.
  • Systemic and pulmonary circulation.
  • Ectothermal.
  • Separate sexes; fertilization usually external in frogs and toads and mostly internal in salamanders and caecilians.
  • Most species return to water to reproduce.

Order Apoda - caecilians.

Order Urodela - salamanders and newts.

Order Anura - frogs and toads.

CLASS REPTILIA Snakes, turtles, crocodiles, lizards

  • Body covered with horny scales.
  • Paired limbs, usually with five toes, adapted for many forms of locomotion (climbing, running or paddling); limbs vestigial or absent in snakes and some lizards.
  • Mouth with undifferentiated teeth.
  • Lungs used in respiration, no gills.
  • Three-chambered heart in most (crocodiles have four-chambered heart); ventricle partly divided.
  • Ecothermic, behavior used in thermoregulation.
  • Internal fertilization and separate sexes; adapted to reproduction on land.
  • Eggs covered with shells; extraembryonic membranes (amnion, chorion, yolk sac, and allantois) present; oviparous.

Terrestrial vertebrates (reptile, birds and mammals) are called amniotes because their embryos are enclosed in the amnion.

Reptiles dominated the earth during the Mesozoic (250-65 m.y.a.).

Mass extinction of animals, including dinosaurs, occurred at the end of the Cretaceous (65 m.y.a.).

  • CLASS AVES Birds
  • Feathers and leg scales.
  • Skeleton fully ossified, with air cavities; horny beak with no teeth; single bone in middle ear.
  • Excrete solid waste in the form of uric acid.
  • Digestive track with a crop for storage, gizzard for grinding and proventriculus that secretes gastric juices.
  • Nervous system and senses of vision and hearing well developed.
  • Four-chambered heart.
  • Endothermic.
  • Respiration by slightly expansible lungs with thin air sacs.
  • Lack diaphragm.
  • Sexes separate.
  • Fertilization internal; eggs with much yolk and calcareous shells.
  • Vocal calls and complex songs.
  • Complex behavior, e.g. courtship, migration.

Early birds had reptilian characteristics and probably evolved from reptiles.

There are about 9,000 species of modern birds divided into 27 orders.

Some zoologists think that the class Aves should be a subclass of the Reptilia.

CLASS MAMMALIA

  • Body covered with hair.
  • Skin with sweat, scent, sebaceous and mammary glands.
  • Mouth with differentiated teeth.
  • Four limbs in most, adapted for many forms of locomotion.
  • Four-chambered heart.
  • Respiration system with lungs and larynx.
  • Muscular diaphragm present.
  • Highly developed nervous system and brain.
  • Three ear ossicles.
  • Endothermic.
  • Internal fertilization; mostly viviparous; fetal membranes (amnion, chorion, allantois).
  • Females nourish the young with milk from mammary glands.

Prototheria: egg-laying mammals (monotremes); eggs may be carried in a pouch on the abdomen or kept in a nest.

Metatheria: part of their development occurs in the mother's uterus and part in an external maternal pouch (marsupials).

Eutheria: entire development in utero (placental mammals); the placenta is an organ of exchange that develops between the embryo and the mother.