The First Multicellular Animals

A.  An examination of the structure of sponges
	1.  Sponges have a vase-shaped body.
		a.  inner layer of collar cells (choanocytes)
		b.  outer layer of epidermal-like cells
		c.  between those two layers are:
			1.  spicules--microscleres and macroscleres
			2.  wander amoebocytes
			3.  reproductive cells	
			4.  porocytes--cells that surround the pores
	2.  Layers can not really be called tissues
		a.  Tissues are groups of cells that have similar structure
			and function and are somewhat specialized.
		b.  Sponges cells are differentiated, but retain potential
			to become other cells
		c.  A sponge can be dissociated into a pile of cells by 
			subjecting it to a chelating agent (something)
			that removes the Calcium for the intercellular 
			cement
		d.  Once dissassociated into a pile of cells, the cells
			will de-differentiate and form a new sponge colony
		e.   The term colony is used because the arrangement between
			the cells that make up the individual is looser than
			in an organism that isn't a colony
	3.  Sponges form a new individual by forming an embryo made up of
		a ball of collar cells, with the collars pointing outwards,
		the embryo then infolds upon itself to produce a two layered
		structure with the collar cells not pointing into the 
		spongocoel.
	4.  Because of all the attributes mentioned above, invertebrate 
		zoologists feel that sponges represent an experiment that
		worked, but from which no other groups were derived.  
		Therefore, we must look at other groups for the ancestral
		group.
B.  What is the simplest structure of a multicellular invertebrate with 
	tissues?
	1.  Simplest structure is found in acoels and planula larvae
		a.  acoels are flatworms, planula larvae are found in the
			cnidaria
		b.  both consist of an thin epidermis surrounding a 
			parenchyma, i.e, a solid mass of cells.
		c.  interior layer opens to outside via small pore.
	2.  Embryologic structure associated with rest of invertebrates is
		a hollow sack formed of at least two layers, an inner 
		endoderm and an outer ectoderm (these are embryologic terms).
		Typical embryo starts out as a hollow ball of cells which
		invaginates to form a two layered sack.
C.  The Gastrea theory
	1.  Early embryologists believed that early organism was a hollow
		two layered creature.  Thought that sponge embryo with
		its movement of collar cells from exterior to interior fit
		the general pattern of a hollow ball of cells from which
		one layer invaginates to produced two layered embryo.
	2.  Saw the typical body shape of the cnidaria as representing that
		aspect.  Polyp stage of cnidarian is a cylinder with inner
		and outer layers.  Medusae stage is an upside down polyp
		with a growth in different axis than the polyp.
	3.  Modern biologist reject the gastrea theory of the origin of 
		the different animal groups because mechanisms of embryology
		can be convergent.
D.  Which group therefore is the antecedant group for the invertebrates.
	1.  Proposals oriented around cnidaria and platyhelminths.
	2.  Platyhelminthes more likely because all cnidaria have stinging
		cells (nematocysts) and it seems unlikely that a structure
		found throughout a whole group would be lost in a descendant,
		particularly when the characteristic is so functionally 
		useful.
	3.  Likely all groups derived from ciliates rather than flagellates
		although sponges are most likely derived from the latter 
		group.