The hemocoel is the cavity in which most of the major internal organs of the arthropod body are found. It is filled with the fluid hemolymph (the arthropod equivalent of blood), which is pumped by a heart and which circulates among and bathes the organs directly without the use of blood vessels.
The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.