Deuterostomes: Overview and Characteristics - Ch34

Main Groups of Deuterostomes

Deuterostomes are a major lineage of animals distinguished by unique embryonic development and complex body plans. The two principal groups are Echinodermata and Chordata.

• Echinodermata: Includes sea stars, sea urchins, and related species. These are coelomates with bilaterally symmetric larvae and pentaradially symmetric adults. They possess a unique water vascular system and an endoskeleton made of calcium carbonate.

• Chordata: Includes lancelets, tunicates, and vertebrates. Chordates are coelomates characterized by a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some stage of development.

Additional info: Hemichordata and Xenoturbellida are also minor deuterostome phyla, but are less commonly discussed in introductory biology.

General Features of Deuterostomes

• Include some of the largest-bodied animals.

• Exhibit high morphological complexity and behavioral diversity.

• Key embryonic development features:

  • Radial cleavage: Cells divide parallel or perpendicular to the axis of the embryo.

  • Formation of anus from blastopore: The blastopore becomes the anus, not the mouth.

  • Enterocoelous: Coelom forms from mesodermal pouches.

  • Indeterminate fate of early blastomeres: Early embryonic cells can develop into a complete organism.

Chordates: Defining Characteristics and Subphyla

Four Defining Characteristics of Chordates

Chordates are defined by four key anatomical features present at some stage of their life cycle:

• Notochord: A flexible, supportive rod running along the body, providing structural support.

• Dorsal hollow nerve cord: A tube of nervous tissue located dorsally, which develops into the central nervous system (brain and spinal cord in vertebrates).

• Pharyngeal slits: Openings in the pharynx that function in filter-feeding (invertebrate chordates) or gas exchange (vertebrates).

• Muscular post-anal tail: An extension of the body past the anal opening, aiding in locomotion.

Three Chordate Subphyla

• Cephalochordata (lancelets): Small, marine, mobile suspension feeders. Retain all chordate features throughout life.

• Urochordata (tunicates, sea squirts, salps): Marine, mostly sessile as adults. Retain only pharyngeal slits as adults; possess a tunic and two siphons.

• Vertebrata (vertebrates): Animals with a backbone. Exhibit all chordate features during embryonic development; adults have a vertebral column and cranium.

Evolutionary Steps and Major Vertebrate Groups

Major Steps in Vertebrate Evolution

The evolution of vertebrates involved several key innovations, each documented by the fossil record:

• Development of a bony exoskeleton (~480 million years ago)

• Evolution of jaws (~440 million years ago)

• Formation of a bony endoskeleton (~410 million years ago)

• Limbs capable of movement on land (~365 million years ago)

• Amniotic egg (~345 million years ago)

Four Major Groups of Vertebrata

• Myxini (hagfish): Jawless, marine scavengers with a cartilaginous skull and notochord.

• Petromyzontida (lampreys): Jawless, mostly parasitic, with a cartilaginous skeleton and no true vertebrae.

• Chondrichthyes (cartilaginous fishes): Includes sharks, rays, and chimaeras. Characterized by a cartilaginous skeleton, paired fins, and internal fertilization.

• Osteichthyes (bony fishes and tetrapods): Possess a bony endoskeleton, operculum, and swim bladder (in most species).

Osteichthyes: Major Groups and Features

Major Groups of Osteichthyes

• Actinopterygii (ray-finned fishes): Most diverse group, with fins supported by bony rays. Includes teleosts, which have protrusible jaws and mobile pharyngeal jaws.

• Sarcopterygii (lobe-finned fishes): Fins supported by a linear bone arrangement; includes lungfish and coelacanths. Gave rise to tetrapods.

• Tetrapods: Four-limbed vertebrates, including amphibians, reptiles, birds, and mammals.

Extra-Embryonic Tissues in Amniotes

Four Extra-Embryonic Tissues

Amniotes (reptiles, birds, mammals) possess specialized membranes that support embryonic development:

• Amnion: Encloses the embryo in a protective fluid-filled sac.

• Yolk sac: Provides nutrients to the developing embryo.

• Allantois: Stores waste products and facilitates gas exchange.

• Chorion: Outermost membrane, involved in gas exchange.

Summary Table: Major Deuterostome Groups

Key Terms and Concepts

• Deuterostome: Animal whose anus forms from the blastopore during embryonic development.

• Coelomate: Animal with a true body cavity lined by mesoderm.

• Radial cleavage: Type of embryonic cell division characteristic of deuterostomes.

• Enterocoelous: Coelom formation by outpocketing of the gut.

• Indeterminate cleavage: Early embryonic cells retain the ability to develop into a complete organism.

Important Equations and Diagrams

• Blastopore fate in deuterostomes:

• Coelom formation (enterocoely):

Example: Chordate Features in Humans

• Notochord: Present as an embryonic structure, replaced by vertebral column in adults.

• Dorsal hollow nerve cord: Develops into the brain and spinal cord.

• Pharyngeal slits: Contribute to structures in the ear and throat.

• Post-anal tail: Present in embryos, reduced in adults.