The long, spiniform processes on the carapace of the Zoëæ of the Crabs and _Porcellanæ_ are not to be explained in this way, but their advantage to the larvæ is evident. Thus, for example, if the body of the Zoëa of _Porcellana stellicola_ (Fig. 24), without the processes of the carapace and without the abdomen, which however is not rigidly extensible, is scarcely half a line in length, whilst with the processes it is four lines long, a mouth of eight times the width is necessary in order to swallow the little animal when thus armed.[2] Consequently these processes of the carapace may be regarded as acquired by the Zoëa itself in the struggle for existence. The formation of new limbs beneath the skin of the larvæ is also to be referred to an earlier occurrence of processes which originally took place at a later period. The original course must have been that they sprouted forth in a free form upon the ventral surface of the larva in the next stage after the change of skin; whilst now they are developed before the change of skin, and thus only come into action a stage earlier. In larvæ which, for other reasons, must be regarded as more nearly approaching the primitive form, the original mode usually prevails in this particular also. Thus the caudal feet (the “lateral caudal lamellæ”) are formed freely on the ventral surface in _Euphausia_ and the Prawns with Nauplius-brood, and within the caudal lamellæ in the Prawns with Zoëa-brood, in _Pagurus_ and _Porcellana._ A compression of several stages into one, and thereby an abridgement and simplification of the course of development, is expressed in the simultaneous appearance of several new pairs of limbs. How earlier young states may gradually be completely lost, is shown by _Mysis_ and the Isopoda. In _Mysis_ there is still a trace of the Nauplius-stage; being transferred back to a period when it had not to provide for itself, the Nauplius has become degraded into a mere skin; in _Ligia_ (Figs. 36, 37) this larva-skin has lost the last traces of limbs, and in _Philoscia_ (Fig. 38) it is scarcely demonstrable. Like the spinous processes of the Zoëæ, the chelæ on the penultimate pair of feet of the young _Brachyscelus_ are to be regarded as acquired by the larva itself. The adult animals swim admirably and are not confined to their host; as soon as the specimens of _Chrysaora Blossevillei,_ Less., or _Rhizostoma cruciatum,_ Less., on which they are seated, become the sport of the waves in the neighbourhood of the shore, they escape from them, and are only to be obtained from lively Acalephs. The young are helpless creatures and bad swimmers; a special apparatus for adhesion must be of great service to them.