The development of the Trachomedusae, so far as it is known, shows an actinula-stage which is either free (larval) or passed over in the egg (foetal) as in _Geryonia_; in no case does there appear to be a free planula-stage. The actinula, when free, may multiply by larval budding, but in all cases both the original actinula and all its descendants become converted into medusae, so that there is no alternation of generations. In _Gonionemus_ the actinula becomes attached and polyp-like and reproduces by budding. [Illustration: After Haeckel, _System der Medusen_, by permission of Gustav Fischer. FIG. 64. _Olindias mülleri._] 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_. FIG. 65.--_Aglantha rosea_ (Forbes), a British medusa.] 4. _Ptychogastridae_ (_Pectyllidae_).--As in the preceding, but with suckers on the tentacles. _Ptychogastria_ Allman (=_Pectyllis_), a deep-sea form. 5. _Aglauridae._--Eight radial canals, two, four or eight gonads; tentacles numerous; tentaculocysts free; stomach prolonged into manubrium. _Aglaura_, _Aglantha_ (fig. 65), &c., with eight gonads; _Stauraglaura_ with four; _Persa_ with two. _Amphogona_, hermaphrodite, with male and female gonads on alternating radial canals. 6. _Geryonidae._--Four or six radial canals; gonads band-like; stomach prolonged into a manubrium of great length; tentaculocysts enclosed. _Liriope_, &c., with four radial canals; _Geryonia_, _Carmarina_ (fig. 26), &c., with six. 7. _Halicreidae._--Eight very broad radial canals; ex-umbrella often provided with lateral outgrowths; tentacles differing in size, but in a single row. _Halicreas_.