INTRODUCTION, Vil 
That every segment has its appendage is a law common 
to all Crustacea. In the Stalk-eyed order, the develop- 
ment of the cephalon is carried to a monstrous extent as 
a shield or carapace, covering and protecting, in some 
cases, all the segments of the pereion. When the cara- 
pace is so developed, the necessity for perfect segments 
in the latter does not exist, consequently the dorsal sur- 
face is wanting; but the lateral portion is always present. 
In the Sessile-eyed Crustacea this enlargement of the 
cephalon does not exist, and the absence of a carapace 
permits the development of the dorsal surface of the 
segments of the pereion. A careful examination of the 
appendages of the head will ciearly show the same 
number of segments associated together as is found to 
exist in the macrural forms, consequently the head or 
cephalon in the Sessile-eyed Crustacea homologizes with 
the carapace in those Crustacea that have their eyes 
supported on foot-stalks. Gradually, from the Brachyura, 
it decreases through each succeeding order, and this, 
apparently, in relative degree with the separation of the 
nervous system into separate ganglia, obedient to a 
common law of depreciation, which in the Sessile-eyed 
Crustacea appears to reach a lower limit in the Jsopoda 
than in the Amphipoda. 
The appendages that are supported by the cephalon are 
various in form, and generally associated with the senses. 
The first, or most anterior pair, are the eyes, which, 
from the circumstance of being closely impacted within 
the dermal skeleton, give the name of Sessile-eyed to the 
legion, as above mentioned. ‘This position is not invari- 
ably the case, since in the genera Tanais, Paratanais, 
Apseudes, &c., the eyes are carried on elevated stalks. In 
the Isopoda these organs appear to be more perfectly 
developed than in the Amphipoda, except, perhaps, in 
Hyperina, where their monstrous development deprives 
