| | Phylogeny (Site not responding. Last check: 2007-10-21) |
 | | Advanced thermoreception is found in Pythonidae, which is considered a primitive family, and Crotalinae, a sub-family of the advanced family Viperidae. |
 | | The phylogenetics of the families Pythonidae and Boidae are not well understood, but it seems that thermoreception with free nerve endings near the surface of the skin evolved in Boidae and that speciallized pits were later derived in Pythonidae. |
 | | In Pythonidae, pits occur along the labial scales, with thermosensitive nerve endings under a thin membrane at the base of the pit. |
| instruct1.cit.cornell.edu /courses/bionb424/students2004/bjp27/phylogeny.htm (346 words) |