What is the function of cnidoblasts in cnidarians?
Cnidoblasts (stinging cells) in cnidarians are used for defense and capturing prey by injecting toxins.
Ref: NCERT Class 11 Biology Chapter 4: Animal Kingdom Phylum Porifera and Cnidaria
#cnidarians
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Which of the following statements about cnidarians is incorrect?
Cnidarians have an incomplete digestive system with a single opening serving as both mouth and anus.
Ref: NCERT Class 11 Biology Chapter 4: Animal Kingdom Phylum Porifera and Cnidaria
Which of the following statements about cnidarians is correct?
Cnidarians possess a nerve net, which helps in basic coordination, but they do not have a centralized nervous system.
Ref: NCERT Class 11 Biology Chapter 4: Animal Kingdom Phylum Porifera and Cnidaria
Dominant medusa stage is found in
Cnidarian body forms include sessile polyp cylindrical with oral disc and pelagic medusa bell-shaped. Dominant medusa stage large free-swimming gelatinous with thick mesoglea prominent gonads within gastrodermis, nerve ring with sensory rhopalia found in Scyphozoa true jellyfishes where scyphomedusa develops from scyphistoma via strobilation producing ephyrae larvae and Cubozoa box jellies with cuboidal bell potent venom. Medusa is primary reproductive adult. Hydrozoa mostly polyp dominant except hydromedusae Anthozoa exclusively polypoid lacking medusa adapting mostly benthic predatory lifestyle with varying coloniality and skeletal deposition in reef environments globally widely distributed and ecologically important as predators and habitat builders.
Ref: Campbell Biology, 12th ed., Chapter 33: Scyphozoa dominant medusa; NCERT Animal Kingdom
Nerve net is characteristic of
Diffuse conduction system termed nerve net characterizes primitive nervous organization in Cnidaria and partially Ctenophora consisting interconnected neurons forming two-dimensional plexus in ectoderm and endoderm without brain ganglia conducting impulses slowly. Enables coordinated retraction tentacle movement feeding responses via radial spread. Epitheliomuscular cells innervated contract. Nerve rings exist around bell margin in medusae associated with rhopalia containing statocysts and ocelli. Such net represents early neural evolution stage before cephalization bilateral symmetry and centralization seen in bilaterians with complex brains and rapid directional conduction needed for active locomotion and sensory integration processes and coordinated behaviors essential.
Ref: Campbell Biology, 12th ed., Chapter 33: Cnidaria nerve net; Brusca & Brusca Cnidaria
Stinging organelles in cnidarians are called
Stinging organelles unique to Cnidaria called nematocysts or cnidocysts housed within specialized epidermal cells termed cnidocytes derived from interstitial stem cells concentrated densest on tentacles oral disc. Each nematocyst consists of capsule containing coiled hollow harpoon thread bathed in toxin triggered to evert explosively penetrating prey. Multiple types penetrant glutinant adhesion volvent entangling. Formation involves Golgi synthesis. Distinguishing synapomorphy of phylum Cnidaria giving group name absent in Porifera and Ctenophora which possess adhesive colloblasts not stinging for prey capture mechanisms evolutionarily distinct and highly specialized for predation and defense in cnidarians with toxic venom systems widely studied.
Ref: NCERT Class 11 Biology, Chapter 4: Cnidaria nematocysts cnidocytes; Campbell Biology Chapter 33
Incomplete gut with single opening is characteristic of
Digestive system with single opening functioning as both mouth and anus termed incomplete gut or blind sac. Cnidarians exhibit gastrovascular cavity where food enters through mouth surrounded by tentacles extracellular digestion begins absorption primarily intracellular via phagocytosis residues expelled same aperture. Similar condition seen in Platyhelminthes with branched gut increasing area. Complete gut with two separate openings enables unidirectional transit regional specialization continuous feeding larger body size innovation arising in Nematoda onward. Incomplete gut correlates with radial symmetry relatively low metabolic demands limiting efficient throughput and complexity in diploblastic grade animals with tissue organization and extracellular digestion.
Ref: NCERT Class 11 Biology, Chapter 4: Incomplete gut cnidarians gastrovascular; Campbell Biology
Cnidarians are best described as
Sponge aquiferous system illustrates increasing complexity and filtration efficiency from simplest asconoid with thin wall single central spongocoel entirely lined by choanocytes to syconoid where wall folded into radial canals increasing choanocyte surface to most derived leuconoid where choanocytes restricted to numerous small spherical chambers interconnected by network of incurrent canals bringing water from ostia and excurrent canals draining to osculum through thick mesohyl. Leuconoid maximizes choanocyte surface to volume ratio enables larger body size generates powerful unidirectional flow enhancing filtering capacity. Present in majority Demospongiae Hexactinellida and most freshwater sponges representing pinnacle poriferan filter-feeding adaptation for benthic ecosystems.
Ref: NCERT Class 11 Biology, Chapter 4: Cnidaria diploblastic radial symmetry; Campbell Chapter 33