50 A HISTORY OF RECENT CRUSTACEA 
CHAPTER V 
THE SUB-CLASS MALACOSTRACA 
THE head and trunk are together composed of thirteen, 
or, if an ophthalmic ring be included, of fourteen seg- 
ments. ‘The caudal part or pleon is composed of six seg- 
ments and a telson. The trunk is clearly distinguished 
from the pleon, but some part of it is always more or less 
closely united with the head. ‘To every segment normally 
belongs a pair of jointed appendages. The eyes are either 
pedunculate, and limited to two in number, with rarely a 
pair of accessory ocelli, or they are sessile, and then gene- 
rally two, but sometimes four, or with the components 
variously distributed. There are two pairs of antenna, a 
pair of mandibles, and two pairs of maxille. Of the next 
eight pairs of appendages, from one to three are maxilli- 
peds, organs of the mouth, the remainder, from seven to 
five in number, being prehensile or locomotive. All these 
are typically seven-jointed. Like the second antenne and 
second maxille they may either have or be without an 
exopod on the second joint, and they may also have or be 
without an epipod on the first. The six pairs of appen- 
dages of the pleon, when present, generally have an 
exopod. The last pair almost always differs in character 
from the rest. The paired appendages of the mouth work 
from the sides, the oral aperture itself being fringed by 
the labrum or upper lip above, and the bifid labium or 
lower lip below. A short cesophagus leads up into the 
stomach. ‘The intestinal tube terminates in the under side 
of the telson. The heart which is dorsally placed has 
lateral openings for the entrance of the blood that has been 
oxygenated in the branchiz. These slits are in one, two, 
