Zoology Unit IV: Classification of Animals

Fundamentals of Zoology

Classification of Animals

• The classification of animals is divided into two major groups: invertebrates and vertebrates
• Invertebrates lack a backbone, while vertebrates have a backbone or spinal column

Invertebrates

• Make up more than 97% of all animal species
• Found almost everywhere, from deserts to deep-sea beds
• Lack a skeletal system, resulting in a soft and often flexible body
• Have an open circulatory system and a simple respiratory system
• Often have an external skeleton that protects their soft body, made of chitin

Vertebrates

• Make up around 3% of all animal species
• Possess a well-defined internal skeleton system, including a backbone
• Have more complex and specialized organ systems compared to invertebrates
• Have bilaterally symmetrical bodies and a brain enclosed by a skull

Phylum Chordata

• Characteristics:
  • Notochord (a longitudinal, cartilaginous rod)
  • Dorsal nerve cord (a bundle of nerves running along the back)
  • Pharyngeal slits (openings that allow water to enter the mouth)
  • Post-anal tail (an extension of the body beyond the anus)

Major Phyla of Invertebrates Relevant to Fisheries

• Mollusca
  • Soft-bodied, triploblastic, and bilaterally symmetrical
  • Coelomate, with a thin fleshy envelope or mantle covering the visceral organs
  • Classes: Gastropoda, Bivalvia, and Cephalopoda

Class Gastropoda

• Second-largest class of animals, with 40,000-90,000 living species
• Characterized by:
  • Snails and slugs with a soft body, usually protected by a shell
  • Foot that spans the length of the body
  • Head with a mouth, sensory structures, and eyes

Class Bivalvia

• Characteristics:
  • Clam-shaped, with a pair of shells (valves) connected by a hinge
  • Body laterally compressed, with a soft, flexible mantle
  • Gills for respiration and feeding
  • Foot that is used for burrowing and anchoring

Class Cephalopoda

• Most morphologically and behaviorally complex class in phylum Mollusca
• Characteristics:
  • Completely merged head and foot
  • Ring of arms and/or tentacles surrounding the head
  • Strong beak, radula, and salivary glands
  • Digestive tract with esophagus, stomach, and caecum
  • Ink gland that produces a suspension of melanin

Phylum Arthropoda

• Characteristics:
  • Jointed legs
  • Segmented body
  • Hard exoskeleton made of chitin
  • Compound eyes
  • Reproduce sexually through external fertilization or asexually

Phylum Echinodermata

• Characteristics:
  • Radial symmetry
  • Water vascular system
  • Ability to regenerate lost body parts
  • Classes: Starfish, Sea Urchins, Sand Dollars, and Sea Cucumbers

Class Holothuridae

• Characteristics:
  • Long, worm-like body with pentaradial symmetry
  • Mouth and anus located on opposite poles
  • Five rows of tube feet along the body
  • Ten to 30 branching tentacles surrounding the mouth
  • Single gonad, with most being dioecious### Phylum Echinodermata
• Holothurians (sea cucumbers) crawl using podia or body wall muscles
• Some deep-sea species have elongate podia for walking
• Trap particles and plankton on mucus-covered tentacles
• Secrete mucus from secretory cells and gland cells

Class Echinoidea (Sea Urchins, Sand Dollars, and Heart Urchins)

• Includes sea urchins, sand dollars, and heart urchins
• Mouth (peristome) is on the bottom, and anus (periproct) is on top
• Aristotle's lantern: a five-part jaw apparatus used for breaking apart food and scraping surfaces for algae
• Irregular echinoids do not have a lantern, and their mouth is on the bottom, but anus is shifted to the posterior side or bottom of the test

Class Asteroidea (Sea Stars)

• Characterized by rays or arms surrounding an indistinct central disk
• Arms are hollow, covered with short spines and pedicellariae, and have tube feet for creeping and clinging
• Can lose one or more arms and grow new ones
• Each arm has an eye spot for sensing changes in light, and tube feet for sensing chemicals, water currents, and objects

Phylum Porifera (Sponges)

• Lack true tissues and organs
• Simple, multicellular, aquatic animals
• Sessile (permanently attached to rocks or other submerged objects)
• Skeletal elements are produced by amoebocytes
• Sponges can reproduce asexually by budding or sexually by producing gametes

Phylum Cnidaria (Corals, Sea Anemones, Jellyfishes, and Hydras)

• Characterized by the presence of cnidae
• Cnidarians have a life cycle that includes both sexual and asexual reproduction
• Class Scyphozoa (True Jellyfish)
  • Characterized by a dominant medusa stage
  • Thick, gelatinous mesoglea layer provides buoyancy
  • Tentacles are equipped with cnidocytes for capturing prey and defense
• Class Anthozoa (Corals, Sea Anemones, and Sea Pens)
  • Lack a medusa stage
  • Radial symmetry, tubular bodies, and a central mouth surrounded by tentacles
  • Cnidocytes are used for capturing prey and defense

Major Classes of Vertebrates Relevant to Fisheries

Class Agnatha (Jawless Fish)

• Represents one of the oldest and most primitive groups of vertebrates
• Characterized by the absence of jaws and paired fins
• Have a cartilaginous skeleton
• Breathe through multiple gill pouches
• Examples: lampreys and hagfish

Class Chondrichthyes (Cartilaginous Fish)

• Skeleton is composed of cartilage rather than bone
• Diverse group, including sharks, rays, and skates
• Characterized by streamlined bodies, tough skin, and formidable jaws
• Examples: Great White Shark, Hammerhead Shark, Manta Ray, and Stingrays

Class Osteichthyes (Bony Fish)

• Skeleton is primarily composed of bone
• Diverse group, including both freshwater and marine environments
• Characterized by the presence of swim bladders and scales made of enamel or ganoin
• Examples: salmon, trout, and Clownfish

Aquatic Reptiles

• Crocodilians (crocodiles and alligators)
• Sea turtles (loggerhead and green turtle)
• Marine iguanas (found in the Galápagos Islands)
• Sea snakes (venomous reptiles)
• Ecological roles: contribute to nutrient cycling, regulate prey populations, and maintain biodiversity

Marine Mammals

• Cetaceans (whales, dolphins, and porpoises)
• Pinnipeds (seals, sea lions, and walruses)
• Sirenians (manatees and dugongs)
• Marine otters (sea otters)
• Ecological roles: regulate sea urchin populations, maintain kelp forest ecosystems
• Conservation challenges: habitat degradation, pollution, climate change, and human activities

Learn about the major groups of animals, including invertebrates and vertebrates, and understand the principles of taxonomy and modern classification systems.