PHYLUM ARTHROPODA 



619 



: 



organ. The rectum or proctodcewm (rect.) is dilated ; the dilated 

 portion (rect. ccec.) gives off two pairs of MalpigJiian tubes (mal.). 



In the Pseudoscorpionida the mesenteron, which is bent into a 

 loop, gives off three diverticula; the proctodseum has also a 

 diverticulum. In the Solpugida the mesenteron also gives off 

 diverticula ; the occurrence of Malpighian tubes is doubtful. In 

 the Acarida there are always diverticula, the number and arrange- 

 ment of which vary, 

 connected with the 

 mesenteron. There 

 .are usually two long 

 coiled Malpighian 



<bes. 

 In the Xiphosura, 

 e mouth (Fig. 514, 

 mo.), which is situ- 

 ated some distance 

 behind the anterior 

 extremity of the 

 body, leads into a 

 .suctorial pharynx, fol- 

 lowed by a stomach, 

 which opens into 

 the elongated mesen- 

 teron ; the procto- 

 dseum, a short tube 

 with folded walls, 

 opens on the ex- 

 terior at the posterior 

 -extremity of the ab- 

 domen. Into the 

 mesenteron, as in the 

 Scorpion, open the 

 ducts of a large 

 gland, usually termed 

 the liver (/. liv.). 



A heart is ab- 

 sent in all the Mites 

 with the exception 



of one family. In the other Arachnida it is present, and has 

 the same general form as in the Scorpions, though always more 

 concentrated. 



In the various orders the organs of respiration differ a good 

 deal in their character. In the Pseudoscorpionida they take the 

 form of branching tracheae similar to those of Insects. In the 

 Pedipalpi there are two pulmonary sacs or book-lungs similar to 

 -those of the scorpions. In the Solpugida there is a system of 



FIG. 512. Eurypterus fischeri (Silurian). 

 (From Nicholson and Lyddeker.) 



