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 Edeypterina, 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. 



Class I. Scorpionida. 



Chaeacteeistics. — 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-hooks. 



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 (Pig. 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. 



