Modifications of a tentacle, as in Trachylinae, or developed from the margin of the umbrella, in no connexion with a tentacle (or, if so connected, not producing any modification in the tentacle), as in Leptolinae.
Further, two distinct types of otocyst can be recognized in the Hydro medusae; that of the Leptolinae, in which the entire organ is ectodermal, concrement-cells and all, and the organ is not a tentaculocyst; and that of the Trachylinae, in which the organ is a tentaculocyst, and the concrement-cells are endodermal, derived from the endoderm of the modified tentacle, while the rest of the organ is ectodermal.
Brooks regards these organs as sensory, serving for the sense of balance, and representing a primitive stage of the tentaculocysts of Trachylinae; Linko, on the other hand, finding no nerve-elements connected with them, regards them as digestive (?) in function.
In Trachylinae the development produces always a medusa, and there is no polyp-stage.
The Trachylinae, on the other hand, are above all oceanic forms, and have no polyp-stage, and hence there is typically no alternation in their life-cycle.
Hence the Trachylinae are termed " hypogenetic " medusae to contrast them with the metagenetic Leptolinae.
In Trachylinae the polyp-stage is passed over, and is represented only by the actinula as a transitory embryonic stage.
In Trachylinae also the beginnings of a similar metagenesis can be found.
Of doubtful position, but commonly referred to the Trachylinae, are the two genera of fresh-water medusae, Limnocodium and Limnocnida.
Both these medusae have sense-organs of a peculiar type, which are said to contain an endodermal axis like the sense-organs of Trachylinae, but the fact has recently been called in question for FIG.
Orders: Eleutheroblastea, Hydroidae seu Leptolinae (Sub-orders: Anthomedusae, Leptomedusae), Hydrocorallinae, Graptolitoidea Trachylinae (Suborders: Trachomedusae, Narcomedusae), Siphonophora.