15.5 Echinoderms and Chordates

Echinoderms and chordates are fascinating animal groups with unique features. Echinoderms, like sea stars and urchins, have radial symmetry and a water vascular system. Chordates, including us, have a notochord and dorsal nerve cord.

Both groups are deuterostomes with true coeloms. Echinoderms have spiny skin and can regenerate limbs. Chordates have pharyngeal slits and a post-anal tail. These traits help them survive and thrive in their environments.

Echinoderms

Features and adaptations of echinoderms

• Echinoderms are marine invertebrates that include sea stars (starfish), sea urchins, sand dollars, and sea cucumbers
• Have a unique water vascular system that functions in locomotion, respiration, and feeding
  • Consists of a series of fluid-filled canals and tube feet
  • Tube feet are small, extensible appendages that protrude through the body wall and are used for movement, adhesion, and sensing the environment
• Exhibit radial symmetry, meaning their body parts are arranged around a central axis
  • Most have a five-fold symmetry, with body parts arranged in multiples of five (pentaradial)
  • Radial symmetry is an adaptation to their sessile or slow-moving lifestyle, allowing them to respond to stimuli from any direction
• Many have a spiny or armored skin, which provides protection from predators (sea urchins)
• Some, like sea stars, have the ability to regenerate lost body parts, such as arms
• Have a unique calcium carbonate endoskeleton composed of ossicles, which provides support and protection
• Possess a simple digestive system, with a mouth on the underside and an anus on the upper surface (sea urchins, sand dollars)
• Develop through indirect development, often involving a metamorphosis stage from a free-swimming larva to the adult form

Chordates

Comparison of major chordate groups

• Chordates are a diverse phylum that includes vertebrates and several invertebrate groups
• All chordates share four key features at some point in their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail
• The three main groups of chordates are:
  1. Cephalochordates (lancelets)
     • Small, fish-like marine invertebrates
     • Retain chordate characteristics throughout their lives
  2. Urochordates (tunicates)
     • Marine invertebrates that include sea squirts and salps
     • Possess chordate features during their larval stage but lose them as adults
  3. Vertebrates
     • Animals with backbones, including fish, amphibians, reptiles, birds, and mammals
     • Retain chordate features throughout their lives, with the notochord developing into the vertebral column
• Evolutionary relationships:
  • Cephalochordates are considered the most basal group, sharing many characteristics with the common ancestor of all chordates
  • Urochordates and vertebrates are more closely related to each other than to cephalochordates
  • Vertebrates are the most diverse and evolutionarily derived group of chordates

Functions of defining chordate features

• Notochord:
  • A flexible, rod-like structure that runs along the dorsal side of the body
  • Provides support and helps with locomotion
  • In vertebrates, the notochord is present during embryonic development and is later replaced by the vertebral column
• Dorsal hollow nerve cord:
  • A hollow tube that runs along the dorsal side of the body, above the notochord
  • Develops into the central nervous system (brain and spinal cord) in vertebrates
  • Allows for rapid communication between the brain and body, enabling complex behaviors and responses to stimuli
• Pharyngeal slits:
  • A series of openings in the pharynx (throat) that connect to the outside environment
  • In aquatic chordates (fish), pharyngeal slits are involved in filter feeding and gas exchange
  • In terrestrial vertebrates, pharyngeal slits are present during embryonic development and give rise to various structures, such as the eustachian tubes and tonsils
• Post-anal tail:
  • An extension of the body posterior to the anus
  • Aids in locomotion, particularly in aquatic species (fish)
  • In some vertebrates (humans, apes), the tail is reduced or absent in adults

Shared characteristics of echinoderms and chordates

• Both groups are deuterostomes, characterized by their embryonic development pattern
• Possess a true coelom, a fluid-filled body cavity that houses internal organs
• Adult forms typically display bilateral symmetry, although echinoderms develop radial symmetry secondarily