84 
Guide to Arachnida. 
Table-case 
No. 19. 
Table- 
cases Nos, 
19, 20. 
The members of this order became extinct in Palaeozoic times. 
They have been found chiefly in the Upper Silurian, but are 
known to extend upwards as far as the Carboniferous. They 
were free-swimming forms, probably marine. 
A model of one of the Gigantostraca (. Eurypterus fischeri) is 
exhibited between Table-cases 16 and 17, in the Insect gallery. 
The fossils from which this model has been reconstructed are 
found in limestone of Upper Silurian age on the island of Oesel 
in the Baltic, and are remarkable from the fact that the chitinous 
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 exhibited in Table-case 19. Reference must 
be also made to the large specimens of Pterygotus and to the 
model of Stylonurus, which are placed on the wall (between 
Cases 12 and 13, and 13 and 14) in the Geological Department. 
Division B.—EMBOLOBBANCHIA. 
The grade Embolobranchia contains the air-breathing forms of 
Arachnida, in which respiration is carried on by internal pulmonary 
sacs or tracheal tubes. There are nine orders. 
Order 1. — Scorpiones (Scorpions). 
The members of this order are remarkably uniform in structure. 
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 chelicerae being small and three- 
jointed, whilst the 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 the latter region are narrowed to form the tail, 
which also includes the post-anal sting. A pair of curious comb- 
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 
