Answer: Adult Schistosome worms. They are shown in cross section at a level where they are not in copula. However, if you were to take sections along the entire length of the worms, you would most likely see the female resting within the male's ventral gynecophoral canal.
Features for identification:
First, the location within a blood vessel and that fact that I told you these are 2 adult worms are big clues. The other important feature to note is that they are flatworms (i.e. trematodes or flukes). If you were to uncurl the edges of the male, you would see that it is fairly flat in cross-section. The schistosomes are the only blood flukes, and therefore, the only flatworms that you would see within a blood vessel. Your other main differential would be the adult filarial worms Wucheria and Brugia spp. They may also be found within vessels (lymphatics) but they are nematodes - round worms (i.e. their body is round on cross-section).
Also, bonus points to Alasdair who identified these worms as S. mansoni. The males of this species have conspicuous tuberculations which are useful from distinguishing it from the smooth-tegumented S. japonicum.