The oldest Zoæa is transmuted at its moult into a form known as Megalopa, which is really almost identical with an anomurous Decapod. No Schizopod stage is intercalated, which shews that the development is in many respects greatly abbreviated. The essential characters of the Megalopa are to be found in (1) the reduction of the two anterior maxillipeds, which cease to function as swimming feet, and together with the appendages in front of them assume the adult form; (2) the full functional development of the five ambulatory appendages; (3) the reduction of the forked telson to an oval swimming plate, and the growth in size of the abdominal feet, which become large swimming plates and are at the same time provided with short endopodites which serve to lock the feet of the two sides.
With these essential characters the form of the Megalopa differs considerably in different cases. In some instances (e.g. Carcinus mœnas) the Zoæa spines of the youngest Megalopa are so large that the larva appears almost more like a Zoæa than a Megalopa (Spence Bate, No. [470]). In other cases, e.g. that represented on [fig. 225], the Zoæa spines are still present but much reduced; and the cephalo-thoracic shield has very much the adult form. In other cases again (e.g. Portunus) the Zoæa spines are completely thrown off at the youngest Megalopa stage.
There is a gradual passage from the youngest Megalopa to the adult form by a series of moults.
Fig. 225. Megalopa stage of Crab larva.
Some of the brachyurous Zoæa forms exhibit considerable divergences from the described type, more especially in the armature of the shield. In some forms the spines are altogether absent, e.g. Maja (Couch, No. [474]) and Eurynome. In other forms the frontal spine may be much reduced or absent (Inachus and Achæus). The dorsal spine may also be absent, and in one form described by Dohrn (No. [478]) there is a long frontal spine and two pairs of lateral spines, but no dorsal spine. Both dorsal and frontal spines may attain enormous dimensions and be swollen at their extremities (Dohrn). A form has been described by Claus as Pterocaris in which the cephalo-thoracic shield is laterally expanded into two wing-like processes.
The Zoæa of Porcellana presents on the whole the most remarkable peculiarities and, as might be anticipated from the systematic position of the adult, is in some respects intermediate between the macrurous Zoæa and that of the Brachyura. It is characterized by the oval form of the body, and by the presence of one enormously long frontal spine and two posterior spines. The usual dorsal spine is absent. The tail plate is rounded and has the character of the tail of a macrurous Zoæa, but in the young Zoæa the third pair of maxillipeds is absent and the appendages generally have a brachyurous character. A Megalopa stage is hardly represented, since the adult may almost be regarded as a permanent Megalopa.
Stomatopoda. The history of the larval forms of the Stomatopoda (Squilla etc.) has not unfortunately been thoroughly worked out, but what is known from the researches of Fritz Müller (No. [495]) and Claus (No. [494]) is of very great importance. There are it appears two types, both of which used to be described as adult forms under the respective names Erichthus and Alima.