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 Amphiihoé 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 larve of the parasitic Isopods show 
a wonderful similarity between the larve of families in 
distantly separated orders. b 
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 [sopoda. 
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 
