350 PHYLUM ARTHROPODA. 



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 lamella: or partitions. These lung- 

 books or pulmonary sacs are believed by some to be chambered or 

 plaited tracheae, while Professor Ray Lankester regards them as in- 

 vaginated modifications of gill-books such as Limnhts possesses. 



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 miniature adults, and adhere for some time after 

 birth to the body of the mother. 



In Enscorpio italic us 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 

 Opisthophthalmits 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 Triiobites. 



Order 2. P.SEUDOSCORPIONID.*:. " Book- Scorpions, " e.g. 



Chelifcr, 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 cheliceiTe are minute suckers, the pedipalps 

 like those of scorpions. The abdomen is broad, with ten to eleven 

 segments. They breathe by tubular tracheae, and have spinning glands. 



Order 3. PKinrALn. "Whip-Scorpions, "e.g. Thelyphouits, Phrynits. 



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

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

 from the thorax, and has eleven to twelve segments. The chelicerse 

 are simply clawed, but are poisonous ; the pedipalps are simply clawed 



