Zoology 250 Lecture 3
KEY CHARACTERS ON THE 'TREE OF LIFE'
- 1) CELLULARITY (unicellular vs multicellular) Metazoans are multicellular; differences b/n 'multicellular' & 'colonial unicellular' are subtle
- 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)
- earliest metazoans had no symmetry; radial & bilateral arose later
- 3) CLEAVAGE TYPE (spiral vs radial) Radial cleavage is primitive in the Metazoa, spiral cleavage is a synapomorphy of the Protostomia
- 4) # TISSUE LAYERS (diploblastic vs triploblastic)
- true tissues (several cell types work together at a single function) are a synapomorphy of the Eumetazoa
- early eumetazoans had only 2 tissue layers (ectoderm & endoderm) surrounding a non-cellular middle layer (diploblastic)
- higher metazoans (Bilateria) have 3 tissue layers: ectoderm, endoderm, and extensive middle layer with complex organs (triploblastic)
- 5) 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)
- 6) TYPE OF CILIATION (mono- vs multiciliated) Monociliated cells are primitive; multiciliated cells arose twice (Proto- & Deuterostomia)
- 7) COELOM TYPE (acoelomate vs pseudocoelomate vs coelomate)
- a true 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
- 8) COELOM ORIGIN (schizocoely vs enterocoely) True coeloms arise developmentally via two different methods: schizocoely (splitting of mesenchyme), enterocoely (outpocketing of epithelium)
- 9) 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
Back to Zool 250 Home Page
Copyright © 2000 by A. Richard Palmer. All rights reserved.(revised Mar. 28, 2000)