Re: Why are earthworms faster than planarians?

  First of all I think that it is important for you to know that 
earthworms and planaria belong to two completely different phyla.  The 
earthworms are part of the phylum Annelida (class Oligochaeta)  and  
planaria belong to phylum Platyhelminthes (class Turbellaria).  Both 
planaria and earthworms are free living even though Platyhelmintha is a 
phylum with mostly (80%) parasitic species.  Since both species are free 
living a means for locomotion had to be evolved.  I will first describe 
the locomotion of planaria and then move on to the earthworm.  
  
  Most of the planaria which live in water have an outer body covered with 
cilia (cilia being somewhat similar to little hairs which aid the animal 
in movement).  These cilia are mostly located on the ventral side ("the 
stomach side") rather than on the dorsal side ("the back side").  Most 
species move by secreting mucus from the ventral surface and beating the 
ventral cilia within this mucus.  Since the planaria are flat (they are 
commonly known as flatworms) there is a substantial increase in the 
surface area of the animal in contact with the surface on which the animal 
is moving.  This is of great advantage to the animal since it allows for 
the maximum number of cilia to be in contact with the substrate (another 
word for the surface on which the animal is moving).  Other than ciliary 
movement, planaria are capable of moving in pedal waves and "looping."  
They have longitudinal, circular, dorsoventral and diagonal muscles.  
Pedal waves are small waves of muscular contraction along the animal's 
ventral surface (stomach side).  These waves are unidirectional, moving 
from the anterior (head) to the posterior (the opposite end..."tail").  As 
a wave of contraction moves down the length of the body, small portions of 
the ventral surface are pulled up and away from the substrate.  Before 
this wave reaches any portion of the body, that portion contracts the 
circular muscles and the body is squeezed and thrust forward, while the 
longitudinal muscle contract just behind the wave and pull the body in the 
direction of locomotion.  Another way of movement in some planaria is 
looping.  All of the musculature listed above is brought into play in this 
locomotory movement.  The individual attaches at the anterior end, pulls 
the posterior end forward by contracting longitudinal muscles, attaches at 
the posterior end, releases the anterior end and then thrusts the body 
forward by contracting the circular muscles.  

  Now on to Annelids.  An earthworm has an inner layer of longitudinal 
muscles covered by a layer of well developed circular muscles.  To move, 
earthworm generates a continuous series of contractions and relaxations of 
the circular and longitudinal musculature.  These cycles of contraction 
are known as peristaltic waves.  Remember that when the longitudinal 
muscles are contracted, it is important for the circular muscles to become 
relaxed.  When this happens the worm becomes "short and fat."  On the 
other hand, when the longitudinal muscles are relaxed and circular muscles 
are contacted the worm appears long and thin.  You can see this if you 
observe a living earthworm.  Both sets of muscles work together in helping 
the animal move.  So if, for example, the head is short and fat (circular 
muscles relaxed, longitudinal contracted) a wave of circular contraction 
can be initiated here. As the circular muscles are contracted at the head
it now becomes longer and thinner and therefore moves forward, while the 
"fat" section is passed along the body.  In summary, a wave contraction of 
the circular muscles begins passing down the body, resulting in forward 
extension of the worm.  As the wave continues to travel down the worm's 
body, from segment to segment, a "recovery phase" is initiated anteriorly; 
that is, a wave of longitudinal muscle contractions moves posteriorly, 
shortening and thickening each segment in preparation for the next 
extension phase. 


  Finally, to answer your question, the annelids move faster than planaria 
because their locomotory mechanism is much more effective.  Their muscles 
are moved with great coordination and they move by means of this 
coordination of circular and longitudinal muscle contractions.  The 
planaria, on the other hand, move with either the help of their cilia and 
secreted mucus (which is not as convenient or efficient) or when they do 
use their muscles they move by "thrusting" their body forward (that is 
almost like throwing your front section forward and then pulling the rest 
forward with the contraction of longitudinal muscles).  Both of these 
processes are much more time consuming than the earthworm's coordinated 
movement.  But please remember that each organism has adapted, through 
millions of years of evolution, the mechanism which best suits their way 
of life.  So despite the fact that earthworm's are faster, the planaria 
are also a very successful species and their way of movement is enough for 
them to survive (feed and escape predators!) and reproduce.               


1)Brusca, R. C., and G. J. Brusca. Invertebrates. 1990. Sinauer 
Associates, Sunderland, MA.