October 2, 2008 - Thursday 
Cnidaria and Porifera
Category: School, College, Greek

Zoology - Lab 2
Phylum: Cnidaria

Class: Hydrozoa
Genus: Obelia
Obelia is a dominant polyp, but produces medusa for sexual reproduction. The inner stem like structure is the coenosarc, a hollow tub composed, like the hydra, epidermis, mesoglea, and gastrodermis. It encloses a gastrovascular cavity that is continuous throughout the colony. The outer transparent covering is the nonliving perisarc. A transparent extension of the perisarc is the hydrotheca, which forms a protective cup around the hydranth. Each hydranth has an elevatedhypostome terminating in a mouth and bearing a circle of tentacles around the base. Each tentacle has in its epidermis rings of swellings caused by clusters of cnidocytes, which bearnematocysts. Food taken by the hydranths can pass it to every part of the colony via thecoenosarc. Reproductive gonangia are club-shaped and arise at the junction of the hydranth and coenosarcs. Each gonangium is made up of a hollow continuation of the coenosarcs called theblastostyle. Obelia is found in shallow coastal areas along both coasts of the United States.