Fig. 30. Older Zoëa of the same Prawn, magnified. Fig. 31. Mysis-form of the same Prawn, magnified. Fig. 30. Older Zoëa of the same Prawn, magnified. Fig. 31. _Mysis_-form of the same Prawn, magnified. During this Zoëal period the paired eyes, the segments of the middle-body and abdomen, the posterior maxillipedes, the lateral caudal appendages and the stump-like rudiments of the feet of the middle-body are formed (Fig. 30). The caudal appendages sprout forth like other limbs freely on the ventral surface, whilst in other Prawns, the _Porcellanæ,_ etc., they are produced in the interior of the spatuliform caudal plate. As the feet of the middle-body come into action, simultaneously with other profound changes, the Zoëa passes into the _ Mysis-_ or Schizopod-form (Fig. 31). The antennæ cease to serve for locomotion, their place is taken by the thoracic feet, furnished with long setæ, and by the long abdomen which just before was laboriously dragged along as a useless burden, but now, with its powerful muscles, jerks the animal through the water in a series of lively jumps. The anterior antennæ have lost their long setæ, and by the side of the last (fourth) joint, endowed with olfactory filaments, there appears a second branch, which is at first of a single joint. The previously multi-articulate outer branch of the posterior antennæ has become a simple lamella, the antennal scale of the Prawn; beside this appears the stump-like rudiment of the flagellum, probably as a new formation, the inner branch disappearing entirely. The five new pairs of feet are biramose, the inner branch short and simple, the outer one longer, annulated at the end, furnished with long setæ, and kept, as in _Mysis,_ in constant whirling motion. The heart acquires new fissures, and interior muscular trabeculæ. During the _Mysis_-period, the auditory organs in the basal joint of the anterior antennæ are formed; the inner branches of the first three pairs of feet are developed into chelæ and the two hinder pairs into ambulatory feet; palpi sprout from the mandibles, branchiæ on the thorax, and natatory feet on the abdomen. The spine on the labrum becomes reduced in size. In this way the animal gradually approaches the Prawn-form, in which the median eye has become indistinct, the spine of the labrum, and the outer branches of the cheliferous and ambulatory feet have been lost, the mandibular palpi and the abdominal feet have acquired distinct joints and setæ, and the branchiæ come into action.