Fig. 35.—Portion of colony of Bougainvillea fruticosa, more magnified.
Fig. 36.—The Medusa form of the same species.
Fig. [36] represents the Medusa form of this species, and the development thus described may be regarded as typical of the Hydroida; yet, as already mentioned, the Æginidæ do not present us with any stage corresponding to the fixed condition of Bougainvillea, but, on the contrary, are developed into Medusæ direct from the egg.
On the other hand, there are groups in which the Medusiform stage becomes less and less important.
Fig. 37, Larva of Prawn, Nauplius stage (after F. Müller). 38, Larva of Prawn, more advanced, Zoëa stage.
The great majority of the higher Crustacea go through well-marked metamorphoses. Figs. [37 and 38] represent two stages in the development of the prawn. In the first (Fig. [37]), representing the young animal as it quits the egg, the body is more or less oval and unsegmented; there is a median frontal eye, and three pairs of natatory feet, the first pair simple, while the two posterior are two-branched. Very similar larvæ occur in various other groups of Crustacea. They were at first regarded as mature forms, and O. F. Müller gave them the name of Nauplius. So also, the second or Zoëa form (Fig. [38]) was at first supposed to be a mature animal, until its true nature was discovered by Vaughan Thompson.