Z250 - Lec8 Outline. Cnidaria I (2004)

Zoology 250 (2004)

Phylum CNIDARIA (I):
INTRODUCTION

(Cnidaria study images;
click on tree to see full cladogram)

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1) Well defined phylum of approx. 10,000, mostly marine, predatory species divided into three main classes (Hydrozoa, Scyphozoa, Anthozoa)
2) Two body forms predominate: polyp (benthic) and medusae (pelagic), alternation of generations is widespread (not haplo-diploid as in plants)
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 (usually) tentacles surround the end bearing the mouth; tentacles capture prey and transport food to 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, three-layered, tissue level of organization (groups of differentiated cells working together for a single function)
- a) ectodermis (outer) and endodermis (inner) are true tissues
- b) the middle layer (mesoglea) is mostly non-cellular and 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 and operculum)
  - cnidocil (sensory cilium= "hair trigger")
6) Nematocysts come in many forms (grappling, penetrating, sticky); one to several forms may occur in a single individual; they aid in feeding (mainly), and also in defense & aggression
7) Are only a few other cell types; most occur in ecto- and endoderm:
- a) epithilio-muscular (cells have basal myofibrils; dual function: skin and muscle)
- b) interstitial (totipotent cells; produce gametes and cnidocytes)
- c) receptor (often have sensory cilium)
- d) nerve (form loosely organized net); no contractile elements
- e) gland (mainly in endodermis); no contractile elements
- f) amoeboid (mobile cells in mesoglea; from ectoderm of Scyphozoa)

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