An Online Introduction to the Biology of Animals and Plants





Key Concepts




Section 3

Chapter 9

The Chordates - Introduction






The Animals We Share a Phylum With...



It used to be thought that all of our close relatives would be vertebrates, with backbones.  But a couple of small sea creatures without backbones still shared a number of distinct features with vertebrates, indicating that they were close relatives, so the phylum was expanded and now includes a couple of types of invertebrates in the groups;  the vertebrates, previously a phylum-level group, are now a sub-phylum inside the phylum of chordates.

Here is a picture showing the features that the chordates share.  We are chordates, but don't go looking for all of the traits on your person - many of the features were only present during your embryo days.  This is part of what led to the idea of "ontogeny recapitulates phylogeny," since we do see several "ancestral" traits only in developing embryos.

 - Chordates get their names from a structure called the notochord.  A notochord has two different uses:  in primitive chordates, it's a flexible skeletal rod useful for the motion of swimming;  in vertebrates, it appears very early in embryos and releases chemicals that cause the layers of the ectoderm over it to thicken and roll up, becoming the spinal cord and usually a brain.  Vertebrate notochords break down and are absorbed after they have done that single job.

 - Chordates have a nervous system based upon a single, hollow nerve cord located under the dorsal (back) surface.  Remember, invertebrates - the mollusks, the segmented worms, the arthropods - have double, solid, ventral nerve cords.

 - Chordates have pharyngeal (throat) pouches with arches of arteries in them and gill slits to the outside.  Many chordates develop gills from at least some of these structures, but land vertebrates, although they have them as embryos, modify them into things such as the roof of the mouth or the voice box.  If your embryology mutated and you were born with functioning gills (which couldnt really happen), you'd almost certainly be missing a bunch of important throat structures - and you wouldn't be able to use the gills anyway, since water doesn't hold enough oxygen to sustain a human.

 - Chordates, like many animals, typically have tails, but only chordates have tails behind their anus.  Sometimes these characteristic features are a bit odd.

 - There are several chordate features that have already shown up in other animal groups, although there isn't necessarily any indications that the other animals are closely related to us.  Chordates, like segmented worms, have a circulation system with blood vessels everywhere, a
closed circulation system - although the systems are "flipped," with ventral (belly-side) hearts in chordates and dorsal hearts in the worms.  Also, chordates are basically segmented (metameric) animals, although this repeating pattern often can be seen strongly only as embryos develop.  You can see a hint of segmentation in the construction of the backbone, its attached muscles, and our peripheral nervous systems, though.  Chordates also commonly have






The Invertebrate Chordates - Some Odd, Some Logical



There are three subphyla in the chordates - one, the vertebrates, will be covered next chapter.  The other two have chordate features but no backbones:

The sea squirts or tunicates (technically, urochordates) spend their adult live as sessile creatures, looking like a vase with two openings.  Water is drawn in and forced  through the gill slits - these are filter feeders.  If these odd animals didnt have tadpole-like larvae with all of the basic chordate features, they might not have been placed in this group.

The lancelets (technically, cephalochordates) are much more like the fish some of their ancestors probably evolved into.  They also generally sit on the bottom and filter food through their gill slits, but they are capable of swimming.






Informational Links



An informational page showing much more about the sea squirts than we could go into here.






Click on term to go to it in the text.
Terms are in the order they appear.



Vertebrate / Invertebrate Chordates  
Ontogeny Recapitulates Phylogeny in Chordates  
Spinal Cord  
Chordate Nervous System  
Pharyngeal Pouches  
Arterial Arches  
Gill Slits 
Tail Behind Anus  
Closed Circulation  
Heart Positions  
Invertebrate Chordates - Types  
Sea Squirts / Tunicates (Urochordates)  
Filter Feeding  
Lancelets (Cephalochordates)







Go On to Next Chapter - Vertebrate Chordates






Online Introduction to the Biology of Animals and Plants.

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