TRACHEATA 395 
and the testes communicate with the exterior by means of two 
ducts, which open independently on the posterior face of the 
operculum or seventh segment. Fertilisation is external. 
The propriety of placing Limulus amongst the Arachnida 
has not been universally accepted, but its resemblance to 
Scorpio, even in points of detail, is striking, and the relation- 
ship thus indicated is strengthened by the structure of the 
fossil EuRYPTERINA, which are to some extent intermediate 
between these two orders. They have six pairs of thoracic 
limbs, and the region between the genital plate and the anus 
is divided into a number of segments gradually diminishing 
in size. 
B. THE TRACHEATE GROUP. 
Cuass I. Scorpionida. 
CHARACTERISTICS.— Arachnida with six pairs of appendages on 
the prosoma, of which the first and second pairs are chelate, 
the latter being enlarged. There is a mesosoma of seven 
segments, and a metasoma of five, terminating in a curved 
spine armed with a poison gland. There are four pairs of 
lung-books. 
Scorpions inhabit warm and tropical countries, hiding for 
the most part under stones, etc., during the day, but running 
about after dusk. They usually carry their metasoma turned 
back over the back (Fig. 227). There are several genera: 
Scorpio, Buthus, Androctonus, etc. 
Their body is covered by a cuticle, which in some places 
is thickened and forms stout plates. Six of these have 
coalesced on the dorsal surface of the prosoma and form a 
carapace which bears from three to six pairs of simple eyes. 
Two of these are close together near the median line, the rest 
are arranged along the anterolateral edge. Behind the 
carapace are seven short broad terga belonging to the meso- 
soma. Each of these is connected with its corresponding 
sterna by soft skin; but the last has to some extent fused 
with the seventh sternum, forming a complete chitinous ring. 
The five segments of the metasoma are completely encased in 
chitin, and so is the postanal poison sting. 
