TRACHEA TA 389 



The Branchiate group includes three classes : 



I. EuEYPTEEiNA (Merostomata). II. Tkilobita. 

 III. XiPHOSUEA (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 : 



I. SCOEPIONIDA. V. ArANEIDA. 



II. PSEUDOSCOKPIONIDA. VI. PhALANGIDA. 



III. PeDIPALPI. VII. ACARINA. 



IV. SOLIFUGA. 



The aberrant forms are — 



I. LiNGUATULIDA. II. TARDIGEADA. 



III. Pycnogonida (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 Limulus 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. 



