TYPE ARACHNID A. 437 



a bunch of unbranclied tracliese, and all gradations between 

 these two conditions occur. The tracheae are lined with 

 chitin, which is sometimes thickened to form rings or spiral 

 bands which serve to keep the lumen of the tubes open and 

 thus permit a free passage of air into them. 



The ccelom is filled for the most part with the various 

 organs and is reduced to a series of lacunar spaces containing 

 blood, sometimes rich in hsemocyanin and assuming a blue 

 color when oxygenated. A heart is wanting in some Mites, 

 but is present in the majority of forms, varying from a saclike 

 organ with a single pair of ostia guarded by valves to an 

 elongated cylinder with as many as eight pairs of ostia (Scorpi- 

 ons). It is for the most part situated in the abdominal region, 

 and in the Spiders is enclosed within a space with definite 

 walls which is termed the pericardium, though it cannot be 

 considered homologous with the pericardium of the MoUusca, 

 since it contains blood ; muscle-bands extend from it to the 

 walls of the body and by their contraction cause its expan- 

 sion, fibres in its wall diminishing its cavity and forcing the 

 blood through the ostia into the heart. Arteries in many 

 forms arise from the heart, but after usually a short course 

 open into the lacunar ccBlom. 



The digestive tract pursues a more or less straight course 

 through the body, but shows a tendency to develop ccecal out- 

 growths which sometimes reach a considerable size. . The 

 anterior and posterior portions of the tract are ectodermal, 

 while the middle region or mid-gut is endodermal and is the 

 portion with which the coeca are connected. In the Scorpi- 

 ons the ducts of a digestive gland open into the mid-gut, and 

 in many forms there is connected with the posteiior portion 

 of this same region a pair of tubular Malpighian vessels 

 which are presumably excretory in function and recall the 

 similar structures of the Amphipoda. The end-gut is fre- 

 quently dilated into a large bladderlike structure, the rectal 

 bladder. 



The nervous system consists of a supraoesophageal syn- 

 cerebrum composed of three pairs of ganglia fused together, 

 and in some forms even four pairs may be included, since the 

 chelicersB may be innervated from the mass, their ganglia in 



