Glossary: Key Terms in Animal Kingdom

Term	Definition	Context
Acoelomate	Animal with no body cavity; space filled with mesenchyme (parenchyma)	Platyhelminthes
Asymmetry	No plane of symmetry can divide animal into equal halves	Most Porifera
Bilateral symmetry	Only ONE plane (sagittal) divides animal into mirror-image halves	Platyhelminthes → Chordata
Bioluminescence	Production of light by living organisms via luciferin-luciferase reaction	Ctenophora
Cephalization	Concentration of nervous tissue and sense organs at the anterior end	Bilaterally symmetrical animals
Chitin	Tough, N-containing polysaccharide forming arthropod exoskeleton	Arthropoda
Choanocytes	Collar cells lining the spongocoel of sponges; create water current and filter food	Porifera
Cloaca	Common opening for digestive, urinary, and reproductive systems	Reptilia, Aves
Cnidocyte	Specialized stinging cell containing nematocyst, unique to Cnidaria	Cnidaria
Coelom	Fluid-filled body cavity lined by mesoderm (peritoneum) on both sides	Annelida → Chordata
Ctenes (comb plates)	Rows of fused cilia used for locomotion in Ctenophora	Ctenophora
Diploblastic	Body wall derived from only 2 germ layers (ectoderm + endoderm)	Cnidaria, Ctenophora
Ecdysis	Periodic moulting/shedding of old exoskeleton to allow growth	Arthropoda
Ectoderm	Outermost germ layer; forms epidermis, nervous system	All triploblastic animals
Ectothermy	Body temperature regulated by external environment (cold-blooded)	Fish, Amphibia, Reptilia
Endoderm	Innermost germ layer; forms gut lining, liver, pancreas	All triploblastic animals
Endothermy	Internal regulation of body temperature (warm-blooded)	Aves, Mammalia
Gastrovascular cavity	Single-opening cavity serving both digestion and circulation	Cnidaria
Haemocoel	Blood-filled body cavity in open circulatory systems	Arthropoda, most Mollusca
Hirudin	Anticoagulant protein secreted by leeches; inhibits thrombin	Annelida (leeches)
Mantle	Fleshy outgrowth in Mollusca that secretes the shell	Mollusca
Medusa	Free-swimming, bell-shaped form of Cnidaria	Cnidaria
Mesenchyme	Loose cellular tissue filling body space in acoelomates	Platyhelminthes
Mesoderm	Middle germ layer; forms muscles, connective tissue, circulatory system	Triploblastic animals
Metameric segmentation	True segmentation with serial repetition of similar body units	Annelida
Metagenesis	Alternation between sexual and asexual generations (polyp ↔ medusa)	Cnidaria
Nematocyst	Coiled thread enclosed in cnidocyte; discharged to capture prey/defend	Cnidaria
Nephridia	Segmental tubular excretory organs in Annelida	Annelida
Notochord	Flexible supporting rod derived from mesoderm; defines Chordata	Chordata
Osculum	Large opening through which filtered water exits a sponge	Porifera
Ostia	Small pores through which water enters a sponge	Porifera
Placoid scales	Tooth-like dermal denticles; oldest type of fish scales	Chondrichthyes
Pneumatic bones	Hollow, air-filled bones reducing weight for flight	Aves
Polymorphism	Occurrence of more than one structural form in the same species	Cnidaria
Polyp	Sessile, cylindrical form of Cnidaria	Cnidaria
Post-anal tail	Extension of body behind the anus; chordate feature	Chordata
Pseudocoelom	False coelom; cavity between mesoderm and endoderm; not fully peritoneum-lined	Aschelminthes
Radula	Rasping, ribbon-like feeding organ with rows of teeth	Mollusca (except bivalves)
Radial symmetry	Multiple planes through central axis divide animal into equal halves	Cnidaria, Ctenophora, adult Echinodermata
Spongin	Flexible protein fibres forming skeleton in some sponges	Porifera (Demospongiae)
Spongocoel	Central cavity of a sponge lined by choanocytes	Porifera
Spicules	Skeletal elements of sponges; $CaCO_{3}$ (calcareous) or $SiO_{2}$ (siliceous)	Porifera
Stomochord	Rudimentary buccal diverticulum in Hemichordata; not a true notochord	Hemichordata
Swim bladder	Gas-filled organ for buoyancy in bony fish	Osteichthyes
Tagmatization	Fusion of similar segments into functional groups (tagmata)	Arthropoda
Triploblastic	Body wall derived from 3 germ layers (ectoderm, mesoderm, endoderm)	Platyhelminthes → Chordata
Tube feet (podia)	Hydraulic extensions of the water vascular system in Echinodermata	Echinodermata
Water vascular system	Hydraulic system of fluid-filled canals connecting to tube feet	Echinodermata exclusively