366 PHYLUM ARTHROPOD A. 



reuniting in a ventral artery above the nerve-cord. From capillaries 

 the blood is gathered into a ventral venous sinus, is purified in the 

 lung-books, and thence returns by veins to the pericardium, finding 

 its way by valved lateral openings (ostia) into the anterior end of each 

 heart-chamber. 



On the ninth to twelfth segments lie slit- like stigmata, the openings 

 of four pairs of lung-books. Each lung-book is like a little purse with 

 numerous (over a hundred) compartments. Air fills the much-divided 

 cavity, and blood circulates in the lamellae or partitions. 



The testes consist of two pairs of longitudinal tubes, united by cross 

 bridges; the vas deferens, with a terminal copulatory modification, 

 opens under the operculum on the first abdominal segment. The 

 ovary consists of three longitudinal tubes, united by cross ducts, and 

 two oviducts open on the under surface of the operculum. 



Fertilisation is internal ; the ova begin their development in the 

 ovary, and complete it in the oviduct. The segmentation is discoidal, 

 the ova are hatched within the mother. The young, thus born " vivi- 

 parously," are like miniatures of the adults, and adhere for some time 

 after birth to the body of the mother. 



In Euscorpio italicus there is abundant yolk in the ovum ; in Scorpio 

 there is little ; but the embryo of the latter seems to eat the terminal 

 part of the ovarian tube in which it develops. In the embryo of 

 Opisthophthalmus there are peculiar horn-like outgrowths, possibly 

 absorptive in function. 



The race of scorpions is of very ancient origin, for one 

 has been found in Silurian strata, and others nearly resem- 

 bling those now alive are found in the Carboniferous. 



In many ways, e.g. in their appendages, endosternite, 

 and coxal glands, the scorpions link the Arachnoids to the 

 King-crabs, and thus to the Trilobites. 



Order 2. PsEUDOSCORPiONiD^E. "Book-Scorpions,"*.^. 

 Chelifer, Chernes 



Minute animals, most abundant in warm climates, under bark, in 

 books, under the wing-covers of insects, etc. They are like miniature 

 scorpions, but without the long tail and sting. Their food probably 

 consists of the juices of insects. There is a cephalothorax with six 

 pairs of appendages; the chelicerse are minute and chelate, with 

 openings of spinning glands, the pedipalps like those of scorpions. 

 The abdomen is broad, with ten to eleven segments. They breathe 

 by tubular tracheae. 



Order 3. PEDIPALPI. "Whip-Scorpions," e.g. Thelyphonus, Phrynm 



Small animals, found in warm countries. There is a cephalothorax 

 with six pairs of appendages ; the abdomen is depressed, well-defined 



