Sub-topic #1: Defining Characteristics of Hydrozoa

• Polyp

  • structural form in which the Cnidarian body is shaped like a hollow cylinder that opens and closes at the top
• Polyp

  • form attaches to hard surfaces and are sessile
• Medusa

  • another structural form of Cnidarias in which they are shaped like an umbrella
• Medusa

  • Not sessile as they swim freely in the ocean.
• Mesentery

  • septum or membrane that divides the gastrovascular cavity
• The medusa stage of hydrozoa is with velum
• Velum

  • shelf of tissue around the bottom of the bell that allows jet propulsion
• Velum

  • serves as site for reproductive cycle of animal
• Ocelli - photo receptor for light sensitivity
• They can only do vertical movements
• Their life cycle is both with polyp and medusa in many species
• Their gastrovascular cavity is not divided by mesenteries, meaning, their gastrovascular cavity is whole
• Their medusa stage is with velum
• Their nematocyst is ectodermal as it is found in the epidermis only
• Their gonads are ectodermal as it develops from cells of the epidermis
• Their medusa are usually with statocysts and ocelli