308 



MANUAL OF ZOOLOGY. 



converging obliquely towards one another, but not joined at 

 their apices " (Owen). 



The first segment of the head bears the compound eyes, 

 which are supported upon long and movable eye -stalks or 

 peduncles. Behind these come two pairs of jointed tactile 

 organs, the larger called the " great antennae," the smaller the 

 " antennules." The mouth is situated on the under surface of 

 the front of the head, and is provided from before backwards 

 with an upper lip (" labrum "), two " mandibles," two pairs of 

 "maxillae," three pairs of " maxillipedes " or "foot-jaws," and 

 a bifid lower lip, or "metastoma" (fig. 158). The five remain- 



Fig. 157. The common Cray-fish {Astacusfluvi- 

 atilis), viewed from below, a Antennules; b 

 Large antennae ; c Eyes ; d Opening of auditory 

 sac ; e Last pair of foot- jaws ; f One of the great 

 chelae ; g Fifth thoracic limb ; h Swimmerets ; 

 / The last pair of swimmerets ; j The opening 

 of the anus below the telson. 



Fig. 158. Gnathites of the 

 Cray -fish (Astac^cs fluvia- 

 tilis). a Mandibles ; b Max- 

 illae ; c Second pair of max- 

 illae ; d First pair of foot- 

 jaws ; e Second pair of foot- 

 jaws ; f Third pair of foot- 

 jaws. 



ing segments of the thorax carry the five pairs of ambulatory 

 legs, of which the first constitute the great claws, or " chelae ; " 

 the next two pairs are also chelate, though much smaller ; and 

 the last two pairs are terminated by simply pointed extremities. 



