xi PHYLUM ARTHROPODA 655 



post-abdomen is a terminal appendage, the caudal spine or sting, 

 swollen at the base and acutely pointed at the apex, where open 

 the ducts of two poison-glands. The anal opening is situated on 

 the ventral surface of the last segment of the post-abdomen, 

 immediately in front of the sting. 



The aperture of the mouth, which is very small, is at the anterior 

 end of the cephalothorax on its ventral aspect ; a lobe which over- 

 hangs it in front is the Idbrum. On each side of the mouth is a 

 three-jointed appendage the chelicera (Fig. 534, chel.) which is 

 terminated by a chela. Behind these are the very large pincer- 

 claws or pedipalpi (ped)., each composed of six podomeres and 

 terminating in a powerful chela. The basal joint of each pedipalp 

 has a process which bites against the corresponding process of the 

 other pedipalp, these processes thus performing the function of 

 jaws. Following upon the pedipalpi are four pairs of walking 

 legs, each composed of seven podomeres, the last of which is 

 provided with curved and pointed horny claws. The basal segments 

 of the first two pairs of walking legs are modified so as to perform 

 to some extent the function of jaws. 



All the six pairs of appendages hitherto described the cheli- 

 cerse, the pedipalpi, and the four pairs of walking legs belong to 

 the cephalothorax. The first segment of the pre-abdomen (Fig. 

 534) has a narrow sternum, on which there is a soft rounded 

 median lobe divided by a cleft; this is termed the genital 

 operculum (op.) ; at its base is the opening of the genital duct. To 

 the sternum of the second segment of the pre-abdomen are attached 

 a pair of remarkable appendages of a comb-like shape the 

 pectines (pect.) each consisting of a stem, along the posterior 

 margin of which is a row of narrow processes, somewhat like the 

 teeth of a comb ; the function of these appendages is doubtful, 

 but is probably sensory. The remainder of the segments of the 

 pre-abdomen, and all those of the post-abdomen, are devoid of 

 appendages. The sterna of the third, fourth, fifth, and sixth 

 segments of the pre-abdomen, which are very 

 broad, bear each a pair of oblique slits the 

 stigmata (stig^) leading into the pulmonary 

 sacs. 



In the interior of the cephalothorax, over 

 the nervous system, is a cartilaginous plate 

 the endosternite (Fig. 535) which serves to 

 give attachment to muscles, and is comparable 

 to the cephalic apodeme of Apus (p. 533). FlG . 5 35.-Endo S termte 



All the appendages of the Scorpion are post- S f scorpion. (After 



, . m ff 0. . f Lankestei.) 



oral in position, and the most anterior the 

 chelicerse are probably best regarded as cor- 

 responding to the antennae of the Crayfish, the equivalent of 

 the Crayfish's antennules and of the antennas of the Cockroach 



