INTRODUCTION. Xlii 



complicated series of organs. It is built up of many 

 separate pairs of appendages, those belonging to the 

 higher groups of Crustacea being the most numerous. 

 In the Sessile-eyed orders, the mandibles are separated 

 from the second or posterior pair of antennae by the ven- 

 tral surface of the fourth or mandibular segment, and a 

 protuberance that, from its position, is called the labrum, 

 or anterior lip. 



In the AmpMpoda, the epistome is generally placed 

 vertically, and occasionally produced anteriorly into a 

 sharp spear-like process. In many, however, as also in 

 the Isopoda, it exists as a plate that gives strength and 

 solidity to the fulcrum on which the mandibles rest. 



The labrum is divided into two parts, the lower of 

 which moves on the upper by a slight hinge, and assists 

 in perfecting the shutting of the mouth. The free margin 

 is generally clothed with short hairs^ often of club-shaped 

 and deformed appearance. 



The mandibles are powerful organs, impinging against 

 each other at their extremities, the biting edge being in 

 the median line. In the Sessile-eyed Crustacea, they 

 bear a near resemblance to the same appendages in the 

 larval condition of the highest order of Crustacea. The 

 anterior or biting margin of the mandible is generally 

 divided into several short and strong denticles, though in 

 some genera it is smooth and even. Within the denti- 

 cular margin a second process generally exists, a smaller 

 repetition of the first, and which commonly, when present, 

 is attached by a movable joint. Near the centre of the 

 mandible is a large internally projecting process, that 

 corresponds with and meets a similar process in the 

 opposite mandible, and is evidently adapted for masti- 

 cation, and may with propriety be named the molar 

 tubercle. It forms, generally, with the anterior or 



