xlvi INTRODUCTION. 



The young are generally white, or of a deep orange 

 colour ; in the adult, the colours vary apparently in rela- 

 tion to the presence of light and other surrounding cir- 

 cumstances. Occasionally the males vary in colour from 

 the females. We see in Orchestia a rosy tint frequently 

 ornamenting the great claw, and some other parts. We 

 have also observed in Amphithoe littorea the well- 

 matured males assume a yellowish appearance. This may 

 also be the case in other genera of which we have not had 

 the opportunity of exact observation. 



In Orchestia, the second hand in the larva bears a near 

 resemblance in form to the same appendage in the female 

 a fact that is, we believe, consistent throughout the 

 entire class. The warty development of one of the pos- 

 terior legs also increases with age. 



In Hyperia, the larva bears but little resemblance to 

 the parent. This was first pointed out by M. Milne 

 Edwards, and next by Mr. Gosse. But more extended 

 observations of the forms of these young animals were 

 detailed by us in a memoir published in the " Annals of 

 Natural History for 1861," on some exotic species. Our 

 observations on the larvse of the parasitic Isopods show 

 a wonderful similarity between the larvae of families in 

 distantly separated orders. 



NERVOUS SYSTEM. 



The nervous system was first made out in a general 

 memoir on the subject by MM. Milne Edwards and An- 

 douin. The observations of these authors have since been 

 generally verified by HH. Loven and Bruzelius in the 

 Amphipoda, and Lereboullet in the terrestrial Isopoda. 

 We have also carefully dissected out most of the system 

 in both the genera Talitrus among the Amphipoda, and 

 Ligia among the Isopoda. The plan of the nervous 



