The tendency to be attracted toward heat is called thermotaxis.
Documented cases
Thermotaxis is very well known in C. elegans (see for example Ploff and Sokut 2010) and the slime molds, Meloidogyne incognita (Pline et al 1988). It has also known in Drosophila (Rosenweig et al. 2004) and mosquitoes (see Patterson and Brown 2009 for Aedes Aegypti). It has been observed in lake sculpin (Cottus extensus) larvea (Wurtsbaugh and Neverman 1988) and mammalian sperm cells (Bahat et al. 2003).
There are probably many undocumented species that display thermotaxis but those are the documented cases I could find.
Many of these species are easy to find in biology labs. Sperms cells are very easy to find especially if you are a male but it may take complex methodology to observe thermotaxis on cells. Bigger organisms are of greater interest.
Do you really need another species?
On a side note, I am wondering whether testing your device on a different species is of much interest to you. Testing the your device create 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 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.
But whether the test is a good idea was not part of your question. Also, I am not involved in this research and you may have good reasons of yours of testing your device on a different species.