Lecture 2

• Life is divided into two super-kingdoms: the Prokarya and Eukarya – these are sub-divided into six kingdoms• Each kingdom has a hierarchy of taxa: Phylum, Class, Subclass, Order, Family, Genus and Species• Animal species are given unique binomial names based on their genus and species – e.g, Mytilus edulis• NB: always in italics or underlined, no capital letter on species name• Modern phylogenies (Trees of Life) suggest multiple origins for contemporary animal taxa
• Cnidocytes (nematocytes)• From greek "cnidos” = “stinging nettle”• Contain cnidocysts (nematocysts)• Used for prey capture or defense
• 30 types of cnidae!• Penetrant – largest and most complex, it pierces skin/exoskeleton of prey and injects venomous fluid (hypnotoxin) which paralyzes/kills – nematocysts.• Glutinant – sticky surface used to stick to prey – ptychocysts.• Volvent – smallest, pear-shaped nematocyst, containing short, thick,spineless, smooth and elastic thread tube – when discharged, it tightly coils around prey – spirocysts.• Taxonomical importance.
• Cnidarians contain some of the most potent toxins known.
• Colonial – multiple tiny polyps connected by hydrocauli.• Hollow cavity in middle of polyp extends into hydrocaulus – individuals intimately connected.• Polyps specialised for feeding – cylindrical body with terminal mouth on raised protuberance (hypostome), surrounded by tentacles.• Perisarc – sheath of chitin and proteins surrounding hydrocaulus.• Stolon – anchors colony to hard surface
• Key features of cnidaria hydrozoa• Not always free-living – in many species medusoid stage is solely a sexuallyreproducing bud on surface of hydroid colony (isolated gonad).• Dome-like umbrella ringed by four tentacles.• Stinging cells on tentacles and around mouth.• Mouth leads to stomach cavity – four radial canals.• Nervous system unusually advanced.• Vellum – circular membrane beneath umbrella which helps propel through water.
• Life cycle of cnidaria hydrozoa

  Usually dioecious (separate sexes) – all polyps in colony either male or female.• Gonozooids = reproductive polyps – bud off asexually producing medusae (either male or female).• Species with polyp and medusa generations – medusa stage is sexually reproductive.• Medusae spawn, releasing gametes into sea.
• CNIDARIA SCYPHOZOA Key features/characteristics• “True jellies”.• From Greek “skyphos” = “drinking cup”.• Exclusively marine.• Two life-history phases: medusa and polyp.• Four-part symmetry.• Mesoglea – provides same structuralintegrity as skeleton, 98% water.• No head, skeleton, specialized organs forrespiration/excretion.• Lack a vellum (different from Hydromedusae)– ring of muscle fibers within mesogleaaround rim of dome.
• Swimming in Scyphozoa: muscles contract against the mesoglea and on relaxation, mesoglea restores shape
• Bioluminescence in coronate scyphozoans -> to scare the predators away eg Atolla wyvillei (Atolla jellyfish)
• Cassiopea xamachana (Upside-down jellyfish) ->lives in tropical, shallow water and has lots of symbiotic algae (why it needs to live in these waters)
• CNIDARIA STAUROZOA Key features/characteristics

  “Stalked jellies”.• Attached to seaweeds, rocks, or gravel.• Majority found in boreal or polar, near-shore, shallow waters.• 50 known species – recently new class(used to be with Scyphozoa).• One of least studied Cnidaria groups.
• CNIDARIA CUBOMEDUSAE Key features/characteristics

  • Box jellyfish – due to box-like (i.e., cube-shaped) bell.• 4 tentacles or tentacle clusters – hang from each ‘corner’ – deliver venom.• Velarium present – pulsation creates powerful jet and fast movement.• True eyes – set in clusters at ends of rhopalia (sensory structures).• In some species polypoid structure transforms directly to medusa (no strobila).