INTRODUCTION. xlll 
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 antenne 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 Amphipoda, 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 
