CHAP. XII. EVOLUTION IN CRUSTACEA. 127 



mode usually prevails in this particular also. Thus the 

 caudal feet (the " lateral caudal lamellae ") are formed 

 freely on the ventral surface in Euphausia and the 

 Prawns with Nauplius-brood, and within the caudal 

 lamellae in the Prawns with Zoea-brood, in Pagurus and 

 Porcellana. 



A compression of several stages into one, and there* 

 by an abridgement and simplification of the course of 

 development, is expressed in the simultaneous appear- 

 ance 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 trans- 

 ferred 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 Zoeae, the chelae 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 Bios- 

 sevillei, Less., or Ehizostoma 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. 



