Animal Diversity: Invertebrates and Vertebrates (Echinoderms & Chordates)

Introduction to Animal Diversity

This section explores the diversity of animals, focusing on the major groups of invertebrates and vertebrates, with an emphasis on the phyla Echinodermata and Chordata. These groups are essential for understanding evolutionary relationships and the development of complex body plans in the animal kingdom.

Deuterostomes

Definition and Major Groups

Deuterostomes are animals in which the anus develops from the blastopore during embryonic development, with the mouth forming secondarily. This group includes the phyla Echinodermata and Chordata.

• Key feature: Deuterostome development pattern

• Major groups: Echinoderms and Chordates

Phylum Echinodermata

General Characteristics

Echinoderms are exclusively marine animals known for their unique body plan and physiology. They exhibit secondary radial symmetry as adults and possess a water vascular system for movement and feeding.

• Symmetry: Modified (secondary) radial symmetry in adults; larvae are bilaterally symmetrical

• Nervous system: Simple, with no cephalization

• Endoskeleton: Covered with spines

• Excretion and respiration: By diffusion; no specialized excretory organs

• Autotomy: Ability to intentionally detach and regenerate body parts

• Reproduction: Sexual, with separate sexes and external fertilization

Body Structure and Water Vascular System

The water vascular system is a network of hydraulic canals unique to echinoderms, used for locomotion, feeding, and gas exchange. Tube feet, connected to this system, allow movement and manipulation of food.

• Five-part body plan: Most echinoderms have pentaradial symmetry

• Larval stage: Bilateral symmetry

Main Classes of Echinoderms

Echinoderms are divided into several classes, each with distinct characteristics and representative species.

Class	Examples	Key Characteristics
Asteroidea	Sea stars	Five arms, tube feet, predatory, eversible stomach
Ophiuroidea	Brittle stars	Long, slender arms, tube feet not used for locomotion, filter feeders
Echinoidea	Sea urchins, sand dollars	Spherical or disc-shaped, no arms, movable spines, feed on seaweeds
Crinoidea	Sea lilies, feather stars	Cup-shaped, stalked, feathery arms, filter feeders
Holothuroidea	Sea cucumbers	Cucumber-shaped, no arms, reduced skeleton, browse on sea bottom

Feeding and Regeneration

Many echinoderms, such as sea stars, are predators and can evert their stomachs to digest prey externally. They are also known for their remarkable regenerative abilities.

• Example: Sea stars feeding on mussels by prying open shells and everting their stomachs

Phylum Chordata

Key Characteristics

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

• Notochord: Flexible, supportive rod running along the back

• Dorsal hollow nerve cord: Develops into the central nervous system

• Pharyngeal slits: Openings in the pharynx, used for filter feeding or gas exchange

• Postanal tail: Tail extending beyond the anus

Subphyla of Chordata

• Urochordata (Tunicates): Marine filter feeders; larvae show all chordate features, adults are sessile and lose most chordate traits except pharyngeal slits

• Cephalochordata (Lancelets): Small, fish-like marine animals; retain all chordate features throughout life; filter feeders

• Vertebrata: Animals with a backbone; includes fishes, amphibians, reptiles, birds, and mammals

Summary Table: Major Invertebrate Phyla

The following table summarizes the main features of major invertebrate phyla, including digestive, circulatory, respiratory, excretory, and nervous systems, as well as reproductive strategies and support structures.

Phylum	Digestive System	Circulatory System	Respiratory System	Excretory System	Nervous System	Reproduction	Support
Ctenophora	Complete gut	Absent	Absent	Absent	Nerve net	Sexual (hermaphrodite)	Mesoglea
Porifera	Absent	Absent	Absent	Absent	Absent	Sexual/asexual	Endoskeleton of spicules/collagen
Cnidaria	Gastrovascular cavity	Absent	Absent	Absent	Nerve net	Sexual/asexual	Mesoglea
Platyhelminthes	Gastrovascular cavity	Absent	Absent	Protonephridia	Brain/ganglia	Sexual/asexual	Parenchyma tissue
Rotifera	Complete gut	Absent	Absent	Protonephridia	Brain/nerve cords	Parthenogenetic	Hydrostatic skeleton
Bryozoa/Brachiopoda	Complete gut	Absent/open/closed	Absent	Metanephridia	No brain/nerve ring	Sexual/asexual	Exoskeleton
Mollusca	Complete gut	Open/closed	Gills	Metanephridia	Ganglia/nerve cords	Sexual/asexual	Hydrostatic skeleton/shell
Annelida	Complete gut	Closed	Absent	Metanephridia	Brain/ventral nerve cord	Sexual/asexual	Hydrostatic skeleton
Nematoda	Complete gut	Absent	Absent	Excretory tubules	Brain/nerve cords	Sexual/asexual	Fluid skeleton
Arthropoda	Complete gut	Open	Trachae/gills/book lungs	Excretory glands	Brain/ventral nerve cord	Sexual/asexual	Exoskeleton
Echinodermata	Usually complete gut	Absent	Tube feet/respiratory tree	Absent	No brain/nerve ring	Sexual/asexual	Endoskeleton of plates
Chordata	Complete gut	Closed	Gills/lungs	Kidneys	Well-developed brain	Sexual	Endoskeleton of cartilage/bone

Subphylum Vertebrata (Craniata)

General Features

Vertebrates are chordates with a backbone and a high degree of cephalization. They possess a bony or cartilaginous endoskeleton, a cranium, and complex organ systems.

• Key features: Vertebral column, cranium, endoskeleton, neural crest-derived structures

• Brain regions: Forebrain, midbrain, hindbrain

• Evolution: Originated from cephalochordate-like ancestors; gene duplications (Hox genes) enabled complex body plans

Main Clades of Living Vertebrates

Vertebrates are classified into several major clades, each with unique adaptations and evolutionary significance.

Clade	Examples	Main Characteristics
Cyclostomata	Lampreys, hagfish	Jawless, no appendages, cartilaginous skeleton
Chondrichthyes	Sharks, skates, rays	Cartilaginous skeleton, no swim bladder, internal fertilization
Actinopterygii	Ray-finned fishes	Ossified skeleton, fins supported by rays, swim bladder
Sarcopterygii	Lobe-finned fishes, coelacanths, lungfishes	Ossified skeleton, fleshy fins with bony extensions
Amphibia	Frogs, toads, salamanders	Adults terrestrial, aquatic larvae, metamorphosis, moist skin
Testudines	Turtles	Body encased in shell, no teeth, eggs laid on land
Squamata	Lizards, snakes	Lower jaw not attached to skull, scales
Crocodilia	Crocodiles, alligators	Four-chambered heart, aquatic predators, parental care
Aves	Birds	Feathers, hollow bones, endothermic, four-chambered heart
Mammalia	Mammals	Mammary glands, hair, specialized teeth, endothermic

Cyclostomata: Jawless Fishes

Cyclostomes include hagfish and lampreys, which are primitive, jawless vertebrates. They lack paired fins and have a simple cartilaginous skeleton.

• Hagfish: Lack eyes, jaws, and vertebrae; produce slime as a defense mechanism

• Lampreys: Have a notochord and rudimentary vertebral column; some are parasitic as adults

Chondrichthyes: Cartilaginous Fishes

This class includes sharks, skates, and rays. Their skeletons are made of cartilage rather than bone, and they possess several unique adaptations for predation and survival.

• Key features: Cartilaginous skeleton, electroreceptors, claspers (in males), squalene in liver for buoyancy

• Reproduction: Internal fertilization

Osteichthyes: Bony Fishes

Bony fishes are divided into two main classes: Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes). They have skeletons made of bone and possess a swim bladder for buoyancy.

• Actinopterygii: Fins supported by bony rays, includes most modern fish

• Sarcopterygii: Fleshy, lobed fins with bony extensions; includes coelacanths and lungfishes

Additional info: The evolutionary transition from lobe-finned fishes to tetrapods marks a significant event in vertebrate history, leading to the colonization of land by vertebrates.