142 INVERTEBRATE ANIMALS. 



the true scorpions the end of the abdomen (Fig. 58) is com- 

 posed of a hooked telson, which is perforated for the duct of a 

 poison-gland, situated at its base. It is by means of this that 

 the scorpions sting ; and the poisonous fluid which they secrete 



FIG. 58. Scorpion (reduced). 



is sufficiently powerful to render their wounds troublesome 

 and painful, if not positively dangerous. The mandibles in 

 the scorpions, as already said, are developed into pincers, and 

 the so-called " maxillary palpi " constitute powerful nipping- 

 claws. The respiratory organs are in the form of pulmonary 

 sacs, four on each side, opening on the under surface of the 

 abdomen by as many distinct apertures or spiracles. 



The scorpions live in the warmer regions of the temperate 

 zone and in tropical countries, and are generally found hiding 

 under stones or in crevices of walls. Their sting, though 

 much exaggerated, is certainly capable of producing very un- 

 pleasant symptoms.* 



ORDER IV. ARANEIDA. In this order are the true Spiders, 

 readily distinguished from the insects, with which they are 

 popularly confounded, by having four pairs of legs, as well as 

 by other characters. In all the true spiders (Fig. 59) the seg- 

 ments of the thorax and head are united to form a single mass 

 or cephalo-thorax, to which the soft and unsegmented ab- 

 domen is joined by a constricted stalk, or neck. Respiration 

 is effected by means of pulmonary sacs, usually conjoined with 

 tracheae. The pulmonary sacs are two or four in number, and 

 open on the surface of the abdomen by as many apertures. 



* Nearly allied to the Scorpions are the so-called " Harvest-spiders 11 (Phalangidce), and 

 the diminutive "Book-scorpion" (CheUfer\ which is commonly to be found among old 

 books. 



