442 ZOOLOGY. 



placed in the sub-class Entomostraca, the orders of which 

 are not very closely allied. 



CLASS : ARACHNIDA. 



Arthropoda without antennae, body divided into cephalo- 

 thorax and abdomen; the cephalothorax bears two pairs 

 of gnathopods, ckelicerae and pedipalpi, and four pairs of 

 legs ; abdomen usually without appendages. The organs 

 of respiration when present are either tracheae or book- 

 lungs, except in Limulus the king-crab, in which they 

 have the form of book- gills attached to the last five pairs 

 of appendages of the abdomen. Like the Crustacea the 

 Arachnida possess a large pair of digestive glands, the 

 so-called liver, opening into the mesenteron. 



The Arachnida include the following orders : 



1. Xiphosura (Limulus), 4. Araneida (Spiders), 



2. Scorpionida (Scorpions), 5. Acarida (Mites), 



3. Phalangida (Harvest-men), 

 and some others of less importance. 



The species of Limulus are marine, and occur off the 

 eastern coast of North America and in the East Indies. 

 They are the only living representatives of the marine 

 ancestors of the Arachnida. The cephalothorax is covered 

 by a broad carapace convex on the dorsal side and semi- 

 circular in outline ; the lower surface of the carapace is 

 flat in front and hollow behind, and its posterior angles 

 are produced. The carapace bears dorsally a pair of 

 simple eyes in the median line anteriorly and a pair of 

 lateral compound eyes ; the abdomen or mesosoma is 

 somewhat triangular in shape and consists of seven seg- 

 ments, the first six of which bear broad flat appendages ; 

 the first of these form the operculum, on the posterior 

 surface of which are the genital apertures ; the next five 

 pairs bear the gill-books, and the seventh bears the anus. 

 Behind the anus is a long spine or telson which is rigid 

 and unjointed ; it probably corresponds to the post-anal 

 telson of the scorpion, the anal segment representing the 

 whole of the scorpion's post-abdomen. Inside the body, 

 near the ventral surface, lies a plate of cartilage of meso- 



