Holt and Tattersall, 1905
The 8th thoracic leg in all instances, and the 7th in all genera exceptThysanopoda , are reduced in the number of segments or are rudimentary. The 1st to the 8th thoracic legs carry gills (gill detail) which are retained on the last pair irrespective of the amount of reduction undergone by these appendages. The gills on the anterior thoracic legs have but one main branch, whereas the posterior ones have several. The inner rami of the 1st and 2nd pairs of pleopods in the males are modified, respectively, as a petasma and as an appendix masculina, believed to function during copulation (petasma) (petasma detail). These structures are of great systematic importance. Bell-shaped luminescent organs are found, usually one on each eyestalk, a pair on the base of the 2nd and 7th thoracic appendages and one ventrallly on the 1st four segments of the abdomen between the pleopods (photophores ventral photo). There are some exceptions, notably in the genus Stylocheiron.