84 Guide to Arachnida. 



Table-case The members of this order liecame extinct in Palaeozoic times. 



No. 19. They liave been found chiefly in the Upper Silurian, bvit are 

 known to extend upwards as far as the Carboniferous. They 

 were free-swimming forms, probably marine. 



A model of one of the Gigantostraca (Euryptenis fisclicri) is 

 exhibited between Table-cases 16 and 17, in the Insect gallery. 

 The fossils from which this model lias been reconstructed are 

 found in limestone of Upper Silurian age on the island of Oesel 

 in the Baltic, and are remarkalile from the fact that the cliitinous 

 substance of the outer coat of the animal has been preserved 

 unaltered in chemical and physical composition. It has been 

 possible to dissolve the remains out from the rock and to mount 

 them as microscopic preparations. As a result, it can now be said 

 that the structure of this species is better known than that of any 

 other extinct Arthropod. Specimens and drawings further illus- 

 trating the group are exhiliited in Table-case 19. Eeference must 

 lie also made to the large specimens of Pteri/yotns and to the 

 model of Sti/Iomirus, which are placed on the wall (l)etween 

 Cases 12 and 13, and 13 and 14) in the Geological Department. 



Division l^.—EMBOLOBBANCHTA. 



The grade Embololjrancliia contains the air-ln-eatliing forms of 

 Arachnida, in which respiration is carried on l)y internal pulmonary 

 sacs or tracheal tubes. There are nine orders. 



Order l.-Scorpiones (Scorpions). 



cases Nos. ^he members of this order are remarkably uniform in structure. 



19, 20. The prosoma (" cephalothorax ") is covered by an unsegmented 

 carapace, which bears from two to five lateral eyes, besides the 

 paired median eyes. The first two pairs of appendages are in the 

 form of pincers, the first pair or ciielicerae being small and three- 

 jointed, whilst tlie second, or palps, are very large and have six 

 joints. All four pairs of legs are of the walking type and are 

 furnished with paired movable claws. The mesosoma, like 

 the metasoma, consists of six distinct somites, and the five 

 posterior of tlie latter region are narrowed to form the tail, 

 which also includes the post-anal sting. A pair of curious comli- 

 like organs, the pectines, tactile in function, are present on the 

 lower surface of the second mesosomatic somite. The respiratory 

 organs consist of four pairs of lung-books, the cavities of which 



