• 1) Arthropod coelom is greatly reduced (limited mainly to the gonads)
  
  a) arises embryologically via schizocoely
  b) most of the primary coelom fuses with the epineural sinus yielding an extensive hemocoel

• 2) Arthropods are divided into 4 subphyla based on: a) tagmatization, b) limb form, & c) location of mouth in the embryo & adult:

  Trait	Chelicerata	Trilobitomorpha	Crustacea	Uniramia
  # of TAGMATA	2: cephalothorax (=prosoma), abdomen (=opisthosoma)	3: head, thorax, abdomen (=pygidium)	2: head, trunk (primitive) 3: head, thorax, abdomen (derived)	2: head, trunk (Myriapoda) 3: head, thorax, abdomen (Insecta)
  ANTERIOR LIMBS	(*= mouth lies just anterior to this segment):
  Acron	none	none	none	none
  Segment #1	*Ý chelicerae	antennae	first antennae	antennae
  Segment #2	* pedipalps/legs	* legs	second antennae	(lost)Ý
  Segment #3	legs	legs	* mandible	* mandible
  Segment #4	legs	legs	first maxilla	maxilla
  Segment #5	legs	legs	second maxilla	labium/maxilla
  Segment #6	legs	thorax legs	thorax legs	thorax legs
  Segment #7	abdomen limbs (if present)	thorax legs	thorax legs	thorax legs
  .	Ý mouth position in Pycnogonida only	.	.	Ý transitory in Symphyla
  MOUTH LOCATION	(*= location of mouth; A= acron; 1,2, etc.= segment number):
  in embryo	A*1	1*2	1*2	1*2
  in adult:	A*1 or 1*2	1*2	2*3	1*3

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