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48,000 species, 1400 of which are invertebrates by virtue of not possessing a backbone

Characteristics
 

• Notochord present during at least part of life.  In vertebrates, the notochord is eventually surrounded by cartilaginous tissue.
 

• dorsal, hollow nerve cord
 

• pharyngeal gill silts

• classic Deuterostome developmental characteristics

Subphyla
Diverse morphologically and relationship with vertebrates is not always obvious.

Subphylum Urochordata

• notochord and dorsal nerve cord typically found only in larval form

• tend to be filter feeders

Includes:

Sessile tunicates with tough protein and polysaccharide tunic or test (the Ascidians).

   Notochord reabsorbed at a dramatic metamorphsis
 

The only chordate characteristics retained in adults are the pharyngeal gill silts.
 Is our basic body plan a neotonous condition?

 


 

Tadpole-like zooplankton that secretes a gelatinous 'house' (the Larvaceans). Tail with notochord drives water movement for filter-feeding,  Notochord retained through out life, but adults are considered neotonous.

 
 
 

 Highly transparent, planktonic salps and doliolids (the Thaliaceans).  May form aggregates.

 

• notochord extends beyond nerve chord to anterior of animal and is contractile


 
 
 
• infaunal filter-feeders
• V-shaped segmented muscles (myomeres) performing antagonistic action on each side of the notochord resulting in fish-like swimming.  Once again, segmentation arises.

Phylogeny

Based on one fossil group (the "calcichordates"), an argument has been made that vertebrates evolved from a clacified echinoderm-like ancestor.  However the proponderance of evidence unites the chordates as a single clade, excluding echinoderms.
 

Cephalochordata, rather than Urochordata, appear to be the sister group of Vertebrata.

• Numerous morphological similarities including muscle arrangement, circulation, spinal nerves, nervous system development, and extent of notochord
 

	Circulatory system	Myomeres
Cephalochordata
Vertebrata (fish)

• Analysis of 18S rDNA sequences

• Similarity of the Hox genes controlling anterior nerve cord (brain in vertebrates)

We are often biased in thinking of the  cephlochordate-vertebrate body plan as a late-comer on the evolutionary scene.  However,  cephlochordate-vertebrate ancestor diverged from other invertebrate groups early in the history of animal life,  The Burgess shale fossil Pikaia is thought to be an early chordate because of the presence of chevron shaped myomeres and an anteriorly extending notochord.  Recently, an even earlier possible chordate ( Myllokunmingia fengjiaoa ) was discovered in  Chenjiang formation in China.

http://www-geology.ucdavis.edu/~GEL3/Cathaymyrus.html                                                                             http://www.gs-rc.org/repo/repoe.htm
 

So, what is left is the animals that are not 'invertebrates'
 
 
 
 

Subphylum "Non-invertebrata"
The remaining 'non-invertebrate' groups (the Subphylum Vertebrata) represent a mere one-twenith of the splendid kingdom of animals!
 
 
 
 
 

However, it does contain one species whose activities have impacted the diversity of life more than any other species since the  beginning of life. But this species also has the ability to consciously decide to be stewards over that creation.

 
 
 
 
 
 
 
 
 
 

Lecture Sources: Pechenik. Jan A.  2000. Biology of the Invertebrates.  McGraw-Hill, New York. http://www.ucmp.berkeley.edu/help/taxaform.html University of California Museum of Paleontology. http://www.teaching-biomed.man.ac.uk/bs1999/bs146/biodiversity/metadiv.htm Graham Davison   University of Manchester SCHOOL OF BIOLOGICAL SCIENCES. http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/home/home.htm Glasgow University Zoology Museum http://biodidac.bio.uottawa.ca/ BIODIDAC:A bank of digital resources for teaching biology

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