Acoelomates and Pseudocoelomates

A.  The nature of the body wall in Cnidaria
     1.  Difference between embryologic vs. adult layers.
          a.  Outer layer--ectoderm vs. epidermis 
          b.  Inner layer--endoderm vs. gastrodermis
     2.  In Cnidaria both inner and outer layers have multiple
          functions.  Both layers act as epithelia, with the 
          epidermis for protection and the gastrodermis as a
          digestive layer.  Both layers have muscular fibers in
          the basal portion of the cells.  These layers are at
          angles with respect to the other and allow for contraction
          and expansion of the body column.
     3.  Cnidaria have a middle layer that is essentially non-
          cellular, called the mesoglea.  It consists of fibers of
          a variety of types.  In some Cnidaria, such as the 
          jellyfish, it conists of few fibers and lots of jelly.
     4.  In the corals, the inner body wall is thrown into layers
          called mesenteries.  The whole animal sits in a calcareous
          space and the mesenteries often contain thin partitions
          of calcareous material, septae
B.  The platyhelminthes are the first group to have a distinct 
     third layer, the mesoderm.
     1.  In the acoels, the mesoderm forms a mesenchymal mass, often
          called a parenchyma.  The mouth is a small indentation
          through the epidermis into a pocket of endoderm that is 
          surrounded by the mesoderm.
     2.  In the platyhelminthes that have a gut, the gut is a series
          of blind tubes that pass into the parenchyma.  The 
          parenchyma has a variety of functions, muscle fibers, 
          reproductive structures, etc.
     3.  The creatures are still small enough to make good use of 
          diffusion as a way to get materials distributed and to use
          cilia as a means of locomotion.  They also secrete mucus     
          that enables them to better move over the substrate.
C.  The pseudocoelomates
     1.  The pseudocoelomates are a dead-end group that are 
          constructed differently from the preceeding groups and
          although very functional.  The group has a body cavity,
          but since that space is only lined by mesoderm on one
          side, it is called a pseudocoel.
     2.  Structure of the creatures, many of which are worm-like,
          consists of an epidermis which secretes a cuticles of
          fibers (fibers in at least three layers), mesodermal 
          cells which lie adjacent to the epidermal cells.  These
          cells have contractile fibers in their bases, a vacu-
          olated region that is bathed with the fluid that fills
          the pseudocoel, and extensions that pass to either the
          dorsal or ventral nerve "cords".  The gut is a complete
          tube with mouth and anus that runs from the anterior end
          to the posterior end.  
     3.  Although the muscle run only in one direction, when they 
          contract, very high "hydrostatic" pressures can be 
          produced, partly because the cuticle is not very elastic.
          Animal bends back and forth and can "push off" against the
          edges of its habitat (soil grains, blood vessel walls,
          plant cells, etc.).
     4.  Some of the advantages of a coelomic cavity
          a.  acts as a supportive skeleton (fluid skeleton)
          b.  facilitates diffusion amongst cells, as materials
               can move faster through a fluid, than through
               "solid" cells.
          c.  greater mixing of materials in coelomic fluid
          d.  space for reproductive organs
          e.  larger-sized organisms
          f.  other ways to move