366 PHYLUM ARTHROPODA. 
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 lamellz 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 etalicus 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, eg. in their appendages, endosternite, 
and coxal glands, the scorpions link the Arachnoids to the 
King-crabs, and thus to the Trilobites. 
Order 2. PSEUDOSCORPIONID&. ‘‘ Book-Scorpions,” eg. 
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 chelicerze 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 trachez. 
Order 3. PEDIPALPI. ‘‘ Whip-Scorpions,” e.g. Zhelyphonus, Phrynus 
Small animals, found in warm countries. There is a cephalothorax 
with six pairs of appendages ; the abdomen is depressed, well-defined 
