x PHYLUM ARTHROPODA 257 



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 

 chelicerae, the pedipalpi, and the four pairs of walking legs 

 belong to the cephalothorax. The first segment of the 

 prae-abdomen (Fig. 152) has a narrow sternum, on which 

 there is placed 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 prae-abdomen are attached a pair of 

 remarkable appendages of a comb-like shape, the pectines 

 (pect), each consisting of a stem, along the posterior mar- 

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

 teeth of a comb ; the function of these appendages is doubt- 

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

 of the prae-abdomen, and all those of the post-abdomen, are 

 devoid of appendages. The sterna of the third, fourth, fifth, 

 and sixth segments of the prae-abdomen, which are very 

 broad, bear each a pair of oblique slits the stigmata 

 (stig) leading into the pulmonary sacs. 



All the appendages of the scorpion are post-oral in posi- 

 tion, and the most anterior the chelicerae are probably 

 best regarded as corresponding to the antennae of the cray- 

 fish, the equivalent of the crayfish's antennules and of the 

 antennae of the cockroach not being present. The pedi- 

 palpi would then be the homologues of the mandibles of 

 the insect and the crustacean. 



The organs of respiration in the scorpions are in the form 

 of pulmonary sacs or book-lungs (Fig. 153, put), the stig- 

 mata or external openings of which have already been 

 referred to. Each pulmonary sac is a compressed chamber 

 lined with a thin cuticle. The lining membrane is raised 



