668 



ZOOLOGY 



termed " liver" {hep.), the ducts of which open into it. The rectum 

 or proctodeum (rect.) is dilated ; the dilated portion (red. ccec.) 

 gives off two pairs of Malpighian tuoes (mat.). 



In the Pseudoscorpionida the mesenteron, which is bent into a 

 loop, gives off three diverticula ; the proctodeum 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 

 tubes. 



In the Xiphosura, 

 the mouth (Fig. 552, 

 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- 

 deum, 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 " (I. liv.). 



A heart is absent 



in all the Mites with 



the exception of one family. In the other Arachnida a heart is 



present and has the same general form as in the Scorpions, 



though always more concentrated. e * 



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 oooh-lungs similar to 

 those of the Scorpions. In the Solpugida there is a system of 



Fig, 



550.— Eurypterus fischeri (Silurian). 

 (From Nicholson and Lydekker.) 



