The tendency to be attracted toward heat is called thermotaxis.
Thermotaxis has been documented in
- C. elegans
- Slime molds Meloidogyne incognita
- Mosquito Aedes Aegypti
- Lake sculpin (Cottus extensus) larvea
- Mammalian sperm cells
The best documentation is on C.elegans. There are probably many undocumented species that display thermotaxis but above are the only documented cases I could find.
Many of these species are easy to find in biology labs. Mammalian sperm cells are very easy to find if you are a male but it may take complex methodology to observe thermotaxis on cells. Bigger organisms are of greater interest. Drosophila might be the way to go.
Do you really need to test with another species?
Whether the test is a good idea was not part of your question but I thought I would comment on it anyway!
I am wondering whether testing your device on a different species is of much interest to you. Testing that your device creates a thermal gradient is sure of interest but one species may react very differently to a specific gradient than another one and it feels a priori to me that you'd be waisting your time by testing it on a different species.
I think it would be more interesting to just place thermometers (if you have access to good accurate thermometers) along space and measure temperature to ensure that a gradient exists and make sure that the temperatures and steep of gradient are within the range of what schistosome may experience in nature.
I am not involved in this research and you may have good reasons of yours of testing your device on a different species.