SCORPIOXIDEA. 



509 



grade of all the A.rachnida. The nervous system is composed of a 

 bilolied brain, a large oval ganglionic mass in the thorax, and seven 

 to eight smaller ganglionic swellings in the abdomen, of which the 

 last four belong to the post-abdomen. The visceral nervous ^vstem 

 is represented by a small ganglion, which is placed at the beginning 

 of the oesophagus, connected with the brain by fibres and gives off 

 nerves to the alimentary canal. The principal organs of sense arc 

 the simple eyes. Of these there are from three to six pairs, which 

 are so distributed that the largest pair is situated on the middle of 

 the cephalo-thorax, and the others right and left at the sides of the 

 frontal region. 



The alimentary canal is a narrow straight tube, which is sur- 

 rounded in the prse-abdomen by the large multilobed liver, and opens 

 on the penultimate ring of the ab- 

 domen. Two Malpighian vessels 

 function as excretory organs. 



The circulation is tho most com- 

 plicated in the whole class, but, as 

 in the Decapoda, special blood si- 

 nuses of the body cavity are inserted 

 into the vascular system. The 

 elongated dorsal-vessel, which is 

 divided into eight chambers and 

 is attached by alary muscles, is 

 surrounded by a pericardia 1 sinus, 

 from which it receives the blood 

 through eight pairs of slit-like open- 

 ings, which are capable of being 

 closed. From the heart the blood 

 is driven through an anterior and posterior artery, and through 

 lateral arteries to the organs. The finer ends of the arteries seem 

 to be connected with the commencing veins by capillaries. From 

 the veins the blood is collected in a receptacle on the ventral 

 surface. Thence the blood passes to the respiratory organs, whence 

 it passes by special veins into the pericardia 1 sinus, and so back to 

 the heart. Respiration is effected by means of four pairs of lung 

 sacs, which open to the exterior by four pairs of stigmata on the 

 third to the sixth abdominal segments and are composed of a rela- 

 tively small number of flat tubes. 



The male and female generative organs open on the ventral fate 

 of the first abdominal segment [the median opening being covered 



FIG. 411. Embryo of a Scorpion (after 

 E. Metschnikoff). JT/, Chelicerse ; Et, 

 pedipalpi ; B' to B lv , the four pairs of 

 thoracic ICL'S. There are rudimentary 

 limbs on the abdomen. 



