Phylum Cnidaria

Phylum Cnidaria Radial or biradial symmetry No posterior or anterior ends Instead they have oral (end with the mouth) and aboral ends Mostly marine Over 9,000 species

Characteristics of Cnidarians Tissue level organization Gelatinous mesoglea Between epidermis and gastrodermis Gastrovascular cavity Nervous system in the form of a nerve net Specialized cells (cnidocytes) for defense, feeding, and attachment

The Cnidarian Body Wall Two layers of tissues Ectoderm of the embryo develops into the epidermis (outer layer of the body wall) Endoderm of the embryo develops into the gastrodermis (inner layer of the body wall) Mesoglea- jellylike layer between the epidermis and gastrodermis Usually acellular (lacks cells)

Gastrovascular Cavity Functions in digestion, respiration, metabolic wastes, and discharge of gametes All particles enter and exit the gastrovascular cavity through the mouth

Nervous System Most primitive of all animal nervous systems Nerve cells located in the epidermis Interconnect to form a nerve net React to stimuli

Cnidocytes Specialized cells used for attachment, defense, and feeding Called “stinging cells” Usually has a modified cilium (cnidocil) Operculum covers the capsule of the cnidocyte at one end Cnida- fluid-filled capsule enclosing a hollow tube Most common type is a nematocyst

Structure & Support Hydrostatic Skeleton- water is confined in a cavity to provide an inflated shape Epitheliomuscular cells contract and aid in moving water throughout the body for support

Feeding Diet consists of small crustaceans and small fish Nematocysts entangle and paralyze prey Tentacles shorten and pull the prey to the mouth Cells in the gastrodermis release enzymes to digest food in the gastrovascular cavity Food is distributed to the rest of the body via peristalsis (contractile motions)

Alternation of Generations Two body forms Polyp- asexual and sessile (does not move) Attaches to a substrate Mouth surrounded by food-gathering tentacles Medusa- dioecious and free swimming Tentacles dangle from it’s margins Mouth faces downward Mesoglea more prominent in this form

Locomotion Polyps move by somersaulting or “inchworming” Medusae move by swimming and floating Use water currents to aid in locomotion Move vertically when swimming Jellyfish swimming

Reproduction Mostly dioecious Embryo elongates into a planula Egg and sperm released into the gastrovascular cavity Embryo elongates into a planula Ciliated free-swimming larva Develops into a polyp once it attaches to a substrate Budding is possible from the body wall (medusas develop this way) May also form other polyps from budding