The development of these parts is as follows--In the aboral thickening of epiblast a cavity makes its appearance, the walls of which constitute the rudiment of the otolithic vesicle (fig. 83 B and C, _s.c._). The roof of the cavity is extremely delicate. On each side of it a thickening of cells becomes established, regarded by Kowalevsky as the rudiment of the nervous ganglia. These thickenings appear to give origin to the lateral ciliated plates. The otoliths arise from cells at four separate points at the corners of the ciliated plates opposite the rows of paddles (fig. 84 A, _ot._). [FIG. 84. TWO STAGES IN THE DEVELOPMENT OF PLEUROBRACHIA RHODODACTYLA. (After Agassiz.) _ot._ otolith; _t._ tentacle.] In Pleurobrachia there is at first only one otolith at each corner. The otoliths are gradually transported towards the centre of the vesicle (fig. 84 B, _ot._) and are there attached, though the four leaf-like suspenders do not arise till very late. The otoliths go on increasing in number throughout life. The gelatinous tissue of the Ctenophora appears as a homogeneous layer between the epiblast and the yolk cells, and is probably homologous with the layer formed in the same situation in all other coelenterate forms. Into the layer a number of anastomosing cells, mainly derived from the epiblast, though according to Chun (No. 174) also in part from the hypoblast, make their way. These cells would appear to be mainly, if not entirely (Chun), of a contractile nature. It is probable that the great mass of the gelatinous tissue of the adult is an intercellular substance derived from these cells. The whole of the above changes are completed while the embryo is still enclosed in the egg-capsule. During their accomplishment the oro-anal axis, which was originally very short, increases greatly in length (fig. 83), so that the embryo acquires an oval form similar to that of the adult. The exact period of leaving the egg does not appear to be very constant but the hatching never takes place till the embryo has practically acquired all the organs of the adult. In the majority of types the differences between the just hatched larva and the adult are inconsiderable, and in all cases the larva has a somewhat oval form. In the case of the Tæniatæ (Cestum, etc.), the larva has the characteristic oval form, and the subsequent changes amount almost to a metamorphosis. The larva of the Lobatæ, such as Eucharis, Bolina, etc., can hardly be distinguished from Pleurobrachia, and undergoes therefore considerable changes after hatching. _Eucharis multicornis_ while still in the larval condition is stated by Chun to become sexually mature.