50 A HISTOKY OF HE CENT 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 antennae, a 

 pair of mandibles, and two pairs of maxillas. 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 antennas and 

 second maxillas 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 oesophagus 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 branchias. These slits are in one, two, 



