STRUCTURE OF THE SCORPION. 285 



by some to be modified tracheae, reduced to chambers with 

 very much plaited walls, but Ray Lankester regards them as 

 invaginated modifications of gill-books such as Ltmulus 

 possesses. 



The nervous system conforms to the Arthropod type, and 

 consists of a dorsal supra-cesophageal ganglionic mass or 

 brain, a ventral chain of seven paired ganglia, and the usual 

 oesophageal ring uniting the most anterior pair with the brain 

 above. The sense-organs include a pair of eyes near the 

 middle of the cephalothoracic shield, with others on the 

 anterior margin. Those in both positions are simple, i.e., 

 with one lens each (monomeniscous) ; the central eyes have 

 a vitreous and a retinal layer (diplostichous) ; those on the 

 margin have only one layer (monostichous). 



Scorpions suck the blood of living animals (insects, etc.), 

 which they may kill with their sting or seize with their pedi- 

 palps, and hold close to the mouth by their chelicerae. The 

 pharynx is a suction pump, the alimentary canal is unusu- 

 ally narrow. Several outgrowths from the mid-gut form a 

 digestive gland. A pair of excretory Malpighian tubes grow 

 out from the hind-gut. 



The body-cavity is for the most part filled up with organs, 

 muscles, and connective tissue. A paired ductless coxal 

 gland occurs in relation to the base of the last thoracic 

 legs. 



The blood contains the common respiratory pigment 

 hsemocyanin and amoeboid corpuscles. An eight-chambered 

 dorsal heart, lying within a pericardium, gives off an anterior 

 aorta supplying head and limbs and curving backwards from 

 the head to follow the ventral nerve-cord ; and there is also 

 a posterior aorta, besides some lateral arteries from the 

 heart chambers. From capillaries the blood is gathered into 

 a ventral venous sinus, is purified by the chambered tracheae, 

 and returns by venous channels to the pericardium and 

 thence into the heart. 



The sexes are separate ; the testes consist of two, the 

 single ovary of three, longitudinal tubes united by cross bridges; 

 the male or female ducts open at a single aperture beneath 

 the operculum on the first abdominal segment ; there is a 

 copulatory organ at the end of the vas deferens. The ovum 

 undergoes total segmentation ; the young are developed 



