INTRODUCTION. V3l 



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 clearly 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-e^-ed 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 Isopoda 

 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, 

 Jpseudes, &c., the eyes are carried on elevated stalks. In 

 the Isopoda these organs appear to be more perfectly 

 developed than in the Ainphipoda, except, perhaps, in 

 Hyperina, where their monstrous development deprives 



