Animal Classification and Organization

• The animal kingdom exhibits a variety of forms and structures, yet shares fundamental features that enable classification

Basis of Classification

• Animal classification considers cell arrangement, body symmetry, coelom nature, and digestive/circulatory/reproductive systems

Levels of Organization

• Animalia members are multicellular but display varied cellular organization

Cellular Level

• Sponges show cellular level organization with cells in loose aggregates and some division of labor

Tissue Level

• Coelenterates have cells organized into tissues performing the same function

Order Level

• Platyhelminthes exhibit organ level organization where tissues form specialized organs

Organ System Level

• Echinoderms and Chordates possess organ systems for specific physiological functions
• Digestive systems can be incomplete (single opening) or complete (two openings)
• Circulatory systems are either open (blood bathes tissues directly) or closed (blood circulates in vessels)

Symmetry

• Animals are categorized based on symmetry

Asymmetrical

• Sponges, which cannot be divided into equal halves by any plane

Radial Symmetry

• Seen in coelenterates, ctenophores, and echinoderms
• Any plane passing through the central axis divides the organism into identical halves

Bilateral Symmetry

• Found in annelids and arthropods
• Body can be divided into identical left and right halves in only one plane

Diploblastic and Triploblastic Organization

• Animals are organized with cells arranged in two or three embryonic layers
• Diploblastic animals have an ectoderm and endoderm with mesoglea in between (e.g., coelenterates)
• Triploblastic animals have a mesoderm between the ectoderm and endoderm (e.g., platyhelminthes to chordates)

Coelom

• The presence or absence of a body cavity is important for classification
• Coelomates have a body cavity lined by mesoderm (e.g., annelids, mollusks, arthropods, echinoderms, hemichordates, and chordates)
• Pseudocoelomates have a body cavity with mesoderm in scattered pouches (e.g., aschelminthes)
• Acoelomates lack a body cavity (e.g., platyhelminthes)

Segmentation

• Segmentation involves external and internal division into segments with serial repetition of organs, as seen in earthworms (metamerism)

Notochord

• Notochord is a mesodermally derived rod-like structure formed during embryonic development in some animals
• Chordates possess a notochord, while non-chordates (e.g., porifera to echinoderms) do not

Classification of Animals

• Kingdom Animalia is broadly classified based on fundamental features

Phylum - Porifera

• Commonly known as sponges
• Mostly marine and asymmetrical
• Primitive multicellular animals with cellular level organization
• Possess a water transport or canal system facilitated by ostia (minute pores), spongocoel (central cavity), and osculum (outer pore)
• Choanocytes (collar cells) line the spongocoel and canals, aiding in food gathering and waste removal

Phylum - Coelenterata (Cnidaria)

• Aquatic, mostly marine animals with radial symmetry
• Presence of cnidoblasts or cnidocytes containing nematocysts on tentacles and body, used for anchorage, defense, and prey capture
• Exhibit tissue level organization and diploblastic nature
• Gastro-vascular cavity with a single opening (mouth) on hypostome
• Two basic body forms: polyp (sessile and cylindrical) and medusa (umbrella-shaped and free-swimming)
• Some exhibit alternation of generation (metagenesis), where polyps produce medusae asexually and medusae form polyps sexually (e.g., Obelia)

Phylum - Ctenophora

• Known as sea walnuts or comb jellies, exclusively marine with radial symmetry
• Diploblastic organization at the tissue level
• Eight external rows of ciliated comb plates for locomotion
• Digestion is both extracellular and intracellular
• Bioluminescence is well-marked
• Exhibit sexual reproduction only

Phylum - Platyhelminthes

• Flatworms with dorso-ventrally flattened bodies
• Mostly endoparasites found in animals, including humans
• Bilaterally symmetrical, triploblastic, and acoelomate animals
• Organ level organization
• Parasitic forms have hooks and suckers
• Flame cells for osmoregulation and excretion
• Some, like Planaria, have high regeneration capacity

Phylum - Aschelminthes

• Roundworms with circular cross-section bodies
• Free-living, aquatic, or terrestrial, and also parasitic in plants and animals
• Organ-system level of body organization
• Bilaterally symmetrical, triploblastic, and pseudocoelomate animals
• Complete alimentary canal with a muscular pharynx
• Sexes are separate (dioecious)
• Internal fertilization, with either direct or indirect development

Phylum - Annelida

• Aquatic (marine and freshwater) or terrestrial, free-living, and sometimes parasitic
• Organ-system level of body organization and bilateral symmetry
• Triploblastic, metamerically segmented, and coelomate animals
• Body surface marked into segments or metameres
• Longitudinal and circular muscles for locomotion
• Nephridia help in osmoregulation and excretion
• Paired ganglia connected by lateral nerves to a double ventral nerve cord
• Nereis is dioecious, but earthworms and leeches are monoecious
• Sexual reproduction

Phylum - Arthropoda

• Largest phylum of Animalia including insects
• Organ-system level of body organization
• Bilaterally symmetrical, triploblastic, segmented, and coelomate animals
• Body covered by chitinous exoskeleton, consisting of head, thorax, and abdomen
• Respiratory organs include gills, book gills, book lungs, or tracheal system
• Circulatory system of open type
• Sensory organs like antennae, eyes (compound and simple), and statocysts
• Excretion through malpighian tubules
• Mostly dioecious
• Fertilization usually internal
• Development may be direct or indirect

Phylum - Mollusca

• Second-largest animal phylum
• Terrestrial or aquatic (marine or freshwater) with organ-system level of organization
• Bilaterally symmetrical, triploblastic and coelomate animals
• Body covered by a calcareous shell, unsegmented, and has a distinct head, muscular foot, and visceral hump
• Mantle over the visceral hump
• Mantle cavity with feather-like gills for respiration and excretion
• Anterior head region has sensory tentacles
• File-like rasping organ (radula) for feeding

Phylum - Echinodermata

• Animals with an endoskeleton of calcareous ossicles (spiny-bodied)
• All are marine with organ-system level organization
• Adult echinoderms are radially symmetrical, but larvae are bilaterally symmetrical
• Triploblastic and coelomate animals
• Digestive system is complete with mouth on the lower side and anus on the upper side
• Water vascular system aids locomotion, capture and transport of food, and respiration
• Excretory system is absent, with separate sexes
• Sexual reproduction, usually external fertilization
• Development is indirect with free-swimming larva

Phylum - Hemichordata

• Worm-like marine animals with organ-system level of organization
• Placed as a separate phylum under non-chordata
• Rudimentary structure in the collar region called stomochord
• Bilaterally symmetrical, triploblastic, and coelomate animals
• Body is cylindrical with a proboscis, collar, and trunk

Phylum - Chordata

• Characterized by the presence of a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits
• Exhibit bilateral symmetry, triploblastic nature, and coelomate condition
• Maintain organ-system level of organization
• Notochord is replaced by a cartilaginous or bony vertebral column in adults

Class - Cyclostomata

• Includes living members are ectoparasites on some fishes
• Have an elongated body with circular mouth lacking jaws
• 6-15 pairs of gill slits for respiration
• Body devoid of scales and paired fins
• Cartilaginous cranium and vertebral column
• Circulation is closed
• Migrate to fresh water for spawning and die shortly after

Class - Chondrichthyes

• Marine animals with streamlined bodies
• Cartilaginous endoskeleton
• Mouth located ventrally
• Notochord persists throughout life
• Separate gill slits without operculum
• Absence of an air bladder necessitates constant swimming to avoid sinking
• Internal fertilization, with many being viviparous

Class - Osteichthyes

• Bony endoskeleton
• Streamlined body with a terminal mouth
• Four pairs of gills covered by an operculum
• Skin covered with scales
• Air bladder regulates buoyancy
• External fertilization

Class - Amphibia

• Adaptable to aquatic and terrestrial habitats
• Body divisible into head and trunk, with tail present in some
• Amphibian skin is moist and lacks scales
• Three-chambered heart
• External fertilization and indirect development

Class - Reptilia

• Creeping or crawling mode of locomotion in terrestrial animals
• Body covered by dry and cornified skin with epidermal scales or scutes
• Three-chambered heart and poikilothermous (cold blooded)
• Shed scales as skin cast
• Internal fertilization, oviparous, and direct development

Class - Aves

• Presence of feathers
• Most fly except for flightless birds
• Forelimbs modified into wings
• Pneumatic bones
• Digestive tract with crop and gizzard
• Four-chambered heart
• Warm blooded, and respiration by lungs

Class - Mammalia

• Found in various habitiats
• Presence of mammary glands for nourishing young ones
• Two pairs of limbs adapted for walking, running, climbing, burrowing, swimming, or flying
• External ears (pinnae) and hair are present
• Heart is four-chambered
• Lungs for air intake