opisthobranchs

/OW0 P AH0 S T AA1 B R AA1 N CH S/
noun
  1. 1

    A gastropod with gills behind the heart, formerly thought to belong to a single group.

Source: https://en.wiktionary.org/wiki/opisthobranch, https://en.wiktionary.org/wiki/opisthobranchs