652 



ZOOLOGY 



SECT. 



narrow tubes, the so-called " Malpighian tubes," which are not the 

 homologues of the tubes so named in the Insects, being, with the 

 cloacal sac, of endodermal and not ectodermal derivation. 



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 

 Malpighian tubes 

 which may be fused 

 mesially. 



In the Xiphosura, 

 the mouth (Fig. 562, 

 mo.), which is situated 

 some distance behind 

 the anterior extremity 

 of the body, leads into 

 a suctorial pharynx, 

 followed by a stomach, 

 which opens into the 

 elongated mesenteron ; 

 the proctodseum, a 

 short tube with folded 

 walls, opens on the 

 exterior at the pos- 

 t e r i o r extremity of 

 the abdomen. 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 many of the Mites. 

 In the other Arach- 



nida a heart 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 trachece 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 

 tracheae. In : the Spiders there are either four pulmonary sacs 



FIG. 560. Eurypterus fischeri (Silurian). 

 (From Nicholson and Lydekker.) 



