Class 4. Arachnida. 



281 



a large ventral blood sinus, and passes thence to the lungs, from 

 whicli the now arterial blood returns to the pericardium, and enters- 

 the heart through the ostia ; the heart of the Scorpions is a long 

 tube, divided, as in the Insects, into a series of chambers (eight), each 

 provided with a pair of ostia. In other Arachnida, the heart is- 

 shorter, and has a smaller number of these 

 ostia, the vascular system is less complete, the 

 blood flowing into large sinuses between the 

 organs. As in other Arthropoda, there is a pair 

 of ovaries in the female, a pair of testes 

 in the male; the two glands, whether ovaries 

 or testes, are frequently partially united, and 

 the ducts open by a common aperture, far 

 forward on the ventral surface of the abdomen. 

 In the PhalangiidsB and the Acarinee, tbe gonads 

 are united at one end, the other ends being 

 ■ prolonged into the oviducts (or vasa def erentia) . 

 These soon unite to form a single canal, which 

 thus arises from a circle formed by the genital 

 glands and their two ducts. Sexual dimorphism 

 is frequently displayed. The Arachnida only 

 occasionally undergo a metamorphosis; 

 the newly-hatched animals are generally like 

 the adults, but sometimes the last pair of limbs is wanting. 



Like Insecta, the Arachnida are emphatically terrestrial and 

 fresh-water forms; many are parasitic. Besides the Pycnogonidse, 

 whose position here is not without some doubt, a few of the 

 Acarinse are marine. 



Fig. 229. Sexual 

 apparatus of one of 

 the Phalangiidae. 

 ovary, u swelling of the 

 long oTiduot, op ovi- 

 positor, m retractor 

 muscles of the same. — 

 After Gegenbaur. 



Order 1. Arthrogastra. 



The members of this order, which includes a number of very 

 different forms, are distinguished from the two following orders, 

 in that the abdomen is segmented. The ohelicerse are 

 generally chelate. Respiration is effected by lungs or tracheee. 



1. The Scoi- pious (Scorpionidse) possess a more elongate body than the 

 rest of the Arachnids. The oephalothorax, which is not constricted off from the 

 ahdomen, bears in the middle line, dorsally, two ocelli, and antei-iorly on each 

 side, a small gi-oup (two to five) ; the chelicerae are short, strong chelae ; the 

 pedipalpi, which forcibly recall the large chelae of the Crayfish, are of considerable 

 length (as long as, or longer, than the legs), and each is furnished with strong 

 claws, the four pairs of legs are well developed. Of the thirteen abdominal 

 segments, the last six are much narrower than the anterior, and form a very movable 

 tail (post-abdomen), which the anim.al curls up over the rest of the body so 

 as to can-y it with the tip pointing forwards; this tip, the sting, bears the 

 openings of two poison glands, which he in the anterior swollen portion of the 



