TRACHEATA 389 
The Branchiate group includes three classes : 
J. EuryprertNa (Merostomata). II. TRILOBITA. 
III. XipHosura (Poecilopoda). 
Of these the first two are extinct, whilst the third is 
represented by a single recent genus, Limulus, the king-crab. 
This, like its fossil congeners, is marine. 
The tracheate division of the Arachnida comprises seven 
classes : 
T. SCORPIONIDA. V. ARANEIDA. 
II. PSEUDOSCORPIONIDA. VI, PHALANGIDA. 
II]. PEDIPALPI. VII. ACARINA. 
TV. SOLIFUGA. 
The aberrant forms are— 
I. LINGUATULIDA. II. TARDIGRADA. 
III. Pycnoconipa (Pantopoda). 
Of these the first class is parasitic and the third is marine. 
The Arachnida form a’most heterogeneous assemblage; the 
group affords shelter to a number of forms whose affinities are 
by no means clear, and whose structure is in some cases so 
modified as to possess few or no Arachnid characteristics. 
A, THE BRANCHIATE GROUP. 
CLass III. Xiphosura. 
The very remarkable creature which is popularly known 
as the king-crab, and scientifically as Limulus, is an inhabitant 
of the warmer waters of the east coast of America, the East 
Indian Archipelago, and the western shores of the Pacific. It 
is found in from two to six fathoms of water, moving about in 
the mud or sand, and living chiefly upon certain worms, It 
may attain a length of several feet. 
The body of Zimulus is enclosed in a thick chitinous 
cuticle, the dorsal surface of which forms a large shield or 
carapace. This is produced laterally into two backwardly 
directed processes, and it covers in the prosoma, the anterior 
region of the body, which bears six pairs of appendages. 
Anteriorly near the middle line are a pair of simple eyes, 
and behind and nearer the sides are a pair of compound eyes. 
