Zoology 250 Lecture 8
CNIDARIA (I): INTRODUCTION
- 1) Well defined phylum of ~10,000, mostly marine, predatory species divided into three main classes (Hydrozoa, Scyphozoa, Anthozoa)
- 2) Two general body forms: medusae (pelagic) & polyp (benthic)
- 3) The basic body plan of all cnidarians is similar:
- a) radially symmetrical
- b) the gut has one opening (mouth only, no anus)
- c) hollow tentacles surround the end bearing the mouth
- d) the gastric cavity has associated canals (=gastrovascular cavity)
- e) the body wall has three, well-defined layers
- 4) Cnidarians exhibit a true tissue level of organization (groups of differentiated cells working together for a single function)
- a) ectodermis (outer) & endodermis (inner) are true tissues
- b) the middle layer (mesoglea) is mostly non-cellular & not a true tissue layer, so cnidarians are diploblastic
- 5) This diverse phylum is united by a unique and complex cell type:
- a) a cnidocyte is a cell that bears two important organelles:
- nematocyst (capsule with an inverted coiled tube & operculum)
- cnidocil (sensory cilium= "hair trigger")
- b) nematocysts come in many forms (grappling, penetrating, sticky)
- c) nematocysts aid in feeding (mainly); also defense & aggression
- 6) Are only a few other cell types; most occur in ecto- & endoderm:
- a) epithilio-muscular cells (cells with contractile elements in base)
- b) interstitial cells (totipotent cells)
- c) receptor cells (often have sensory cilium)
- d) nerve cells (form loosely organized net); no contractile elements
- e) gland cells (mainly in endodermis); no contractile elements
- f) amoeboid cells (mobile cells in mesoglea; produced by ectoderm)
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Copyright © 1999 by A. Richard Palmer. All rights reserved.
(revised Jan. 7, 1999)