There is no difficulty whatever in regarding Hydra as bearing the same relation to the actinula-stage of other Hydromedusae that a Rotifer bears to a trochophore-larva or a fish to a tadpole.
B, Side view of the trochophore larva of Eupomatus uncinatus (from Hatschek).
Ment and of the struc ture of the disk to their identification of "trochus" and "cingulum" with the preoral and postoral wreaths of the trochophore larva.
8, D) (which has a male of the same type as Melicerta, &c.) is an extremely modified type, and its resemblance to the trochophore larva of Lepadorhynchus or Polygordius is only superficial.
I, C and D) is a typical but very specialized form of trochophore, provided with a branching nephridium bearing solenocytes.
(From Fraipont.) C, Trochophore of Polygordius.
It is an ancestral larval form, corresponding perhaps to the stages immediately succeeding the trochophore in the development of Annelids, but with some of the later-acquired Crustacean characters superposed upon it.