Zoology 250 Lecture 3
KEY CHARACTERS ON THE 'TREE OF LIFE'
- 1) CELLULARITY (unicellular vs multicellular) Metazoans are all multicellular, but the difference between 'multicellular' and 'colonial unicellular' can be subtle (will review when discussing sponges).
- 2) BODY SYMMETRY (radial vs bilateral)
- radial symmetry refers to symmetry about a single axis, bilateral symmetry refers to symmetry about a plane (defined by two axes)
- the earliest metazoans were radially symmetrical
- 3) # TISSUE LAYERS (diploblastic vs triploblastic)
- true tissues (several cell types work together at a single function) are a synapomorphy of the Eumetazoa
- the earliest eumetazoans had only 2 tissue layers (ectoderm & endoderm) surrounding a non-cellular middle layer (diploblastic)
- higher metazoans (Bilateria) exhibit 3 distinct tissue layers: ectoderm, endoderm, and an extensive middle layer where tissues have become organized into complex organs (triploblastic)
- 4) COELOM TYPE (acoelomate vs pseudocoelomate vs coelomate)
- a coelom is a fluid-filled internal body cavity lined by epithelium
- fluid-filled internal body cavities take several forms and arose independently in several higher groups of invertebrates
- 5) CLEAVAGE TYPE (spiral vs radial) Radial cleavage is primitive in the Metazoa, spiral cleavage is a synapomorphy of the Protostomia.
- 6) TYPE & ORIGIN OF GUT (blind vs complete) A blind gut (mouth, no anus) was primitive in the Eumetazoa; a complete gut (mouth & anus) arose twice independently (Protostomia & Deuterostomia)
- 7) TYPE OF SKELETON (hydrostatic, endoskeleton, exoskeleton) An exoskeleton lies outside the body and is produced by ectoderm, an endoskeleton lies inside the body and is produced by mesoderm or mesoglea, a hydrostatic skeleton is fluid-filled and lies inside the body
- 8) TYPE OF CILIATION (mono- vs multiciliated) Monociliated cells are primitive in the Metazoa, multiciliated cells arose twice independently (Protostomia & Deuterostomia)
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Copyright © 1999 by A. Richard Palmer. All rights reserved.(revised Jan. 6, 1999)