ASTACUS FLUVIATILIS. 265 



being alone visible. These, and the two pairs which lie im- 

 mediately under or in front of them, are called inaxillipedes^ 

 or "foot-jaws." The next two pairs, delicate and foliaceous, 

 are the maxillse ; while beneath or rather in front of them 

 are two strong, toothed organs, the mandibles. These, the 

 maxillae and the maxillipedes, thus constitute six pairs of 

 gnathites. 



The remaining three pairs of appendages occupy the sides 

 of the forepart of the cephalo-thoiax, in front of the moulh. 

 The most posterior pair, or the long feelers, are the antennae; 

 the next, or the short feelers, are the antennulae; while the 

 most anterior pair are the movable stalks, which support the 

 eyes upon their extremities — the " ophthalmic peduncles," or 

 " ophthalmites." 



To arrive at an understanding of the composition of this 

 complex body with its multiform appendages, we must first 

 detach and study carefully one of the abdominal segments — 

 say the third. Such a segment is nearly semi-circular in ver- 

 tical section, the dorsal wall, or tergum, being very convex, 

 and where it reaches the level of the almost straight ventral 

 wall, or sternum, sending down a flattened lobe, which is re- 

 flected at its free edges into a corresponding prolongation of 

 the ventral wall, so that each infero-lateral angle of the seg- 

 ment is prolonged into a hollow process, the pleuron. Near 

 the outer extremities of the straight ventral portion of the 

 segment two rounded articular cavities, which receive the 

 basal joints of the appendages, are situated. A transverse 

 groove will be seen on the tergum, separating rather more 

 than the anterior third of its surface, as a smooth, convex, 

 lenticular facet, which is completely overlapped by the pos- 

 terior margin of the preceding segment, when the abdomen 

 is extended, and is left uncovered only in complete flexion. 

 This is the tergal facet. A corresponding flattened and rath- 

 er excavated surface upon the anterior half of the pleuron, 

 which is similarly overlapped by the preceding pleuron, and 

 is left uncovered only in complete extension, may be termed 

 the pleural facet. It will be observed that there is a close 

 correspondence between the skeleton of an abdominal somite 

 of a Cray-fish, and that of a thoracic somite of a Trilobite ; 

 except that, in the latter, the sternal region is not calcified. 



The appendages of the segment (Fig. 71, K) are very sim- 

 ple, consisting of a cylindrical basal portion, divided into two 

 joints, a shorter proximal, and a longer distal, to the latter of 

 which two terminal many-jointed filaments are articulated. 

 12 



