The Trachomedusae are divided into the following families:
1. _Petasidae_ (_Petachnidae_).--Four radial canals, four gonads;
stomach not prolonged into the manubrium, which is relatively short;
tentaculocysts free. _Petasus_ and other genera make up this family,
founded by Haeckel, but no other naturalist has ever seen them, and it
is probable that they are simply immature forms of other genera.
2. _Olindiadae_, with four radial canals and four gonads; manubrium
short; ring-canals giving off blind centripetal canals; tentaculocysts
enclosed. _Olindias mülleri_ (fig. 64) is a common Mediterranean
species. Other genera are _Aglauropsis_, _Gossea_ and _Gonionemus_;
the last named bears adhesive suckers on the tentacles. Some doubt
attaches to the position of this family. It has been asserted that the
tentaculocysts are entirely ectodermal and that either the family
should be placed amongst the Leptomedusae, or should form, together
with certain Leptomedusae, an entirely distinct order. In
_Gonionemus_, however, the concrement-cells are endodermal.
3. _Trachynemidae._--Eight radial canals, eight gonads, stomach not
prolonged into manubrium; tentaculocysts enclosed. _Rhopalonema_,
_Trachynema_, &c.
[Illustration: After E. T. Browne, _Proc. Zool. Soc. of London_.