Cnidaria Hydrozoa

• 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 (hypos toe) surrounded by tentacles
• Periasarc - sheath of chitin and proteins surround hydrocaulus
• Storon - anchors colony to hard surface
• Medusae (main body form) - umbrella or bell shaped, with tentacles hanging down. Smaller than jellyfish
• Not always free living - in many species medusoid stage is solely a sexually reproducing 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

  Usually dioecious (separate sexes) - al polyps in colony either male or female
• Gonozooias = 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
• Life cycle