Phylum Cnidaria Body Plan Cnidarians have two general

Phylum Cnidaria

Body Plan Cnidarians have two general body forms: • Medusa POLYP • Polyp MEDUSA Both have a body wall made of three layers that surround an internal cavity called the gastrovascular cavity.

The outer layer, epidermis, covers the outer surface of the cnidarian. The middle layer, mesoglea, can be thin or thick. • Thick layer is jelly-like and may have amoebocytes in it. The inner layer, gastrodermis, lines the gastrovascular cavity. • Involved in digestion.

MESOGLEA EPIDERMIS GASTROVASCULAR CAVITY GASTRODERMIS

Form and Function Almost all cnidarians feed using nematocysts. Spring loaded darts which release a neurotoxin. They are located along the tentacles. Tentacles move paralyzed/dead prey through mouth into gastrovascular cavity.

Prey are digested in the gastrovascular cavity. Partially digested food circulated around to tissues in gastrovascular cavity, vaguely like blood. • Digestion completed intracellularly. Undigested materials are passed back out through the mouth. • The digestive tract has one opening only!

Most cnidarians are only a few cell layers thick. • No need for complex organs. • Tissues rely on diffusion for exchange of O 2 and CO 2. No organized internal excretory or transport systems! Lack a brain. • Simple nervous system which is called a nerve net.

Cnidarians have specialized sensory cells which transmit information to the nerve net. • Both in polyps and medusae. There also simple sensory organs. • Only in medusae. 1. Statocysts: Act as balancer organs. 1. Ocelli: Simple eyes that detect light and dark.

STATOCYST with a STATOLITH SIMPLE EYE LOCATED HERE:

Cnidarians have epidermal cells which are contractile. • Meaning they contract like muscles! • Called myoepithelial cells. • Stimulated by the nervous system. Allows movement of the tentacles and the body in polyp and allows medusa to pulsate.

A GOOD SLICE OF EPIDERMAL TISSUE.

Reproduction Most cnidarians can reproduce sexually and asexually. Asexually: Polyps can bud off other polyps or tiny medusae. Sexually: Mostly gonochoristic.

ANSWER KEY Buds off many individuals. Sexual stage Asexual stage What we normally see.

Sexual stage What we normally see. Buds off many individuals. Asexual stage