SCORPIONIDEA. 



803 



arisen more than once in the history of invertebrates. In the 

 scorpion they are said to originate from the mesenteron and 

 are therefore lined by hypoblast, whilst in insects and most 

 arachnids they grow out from the proctodaeum and so are epi- 

 blastic in origin. 



There are however in scorpions and other Arachnids other 

 glands engaged in the same work of ridding the system of nitro- 

 genous excreta. These are the coxal glands. They lie one on 

 each side in the prosoma just above the bases of the 5th and 6th 

 appendages, and they open to the exterior in the young by a 

 minute pore on the posterior face of the coxa of the 5th appen- 



10 



FIG. 522. View of internal anatomy of Buthus, showing digestive, circulatory, respiratory, 

 and nervous systems (from Leuckart, after Newport and Blanchard). 1 Chelicerae ; 2 

 pedipalpi ; 3-6 ambulatory limbs ; 7 pectines ; 8 mesenteron ; 9 lobules of liver, with 

 ducts entering mesenteron ; 10 malpighian tubules, portions of ; 11 proctodaeum ; 12 

 anus ; 13>-13iv lung sacs ; 14 heart ; 15 posterior aorta ; 16 anterior aorta ; 17 brain 

 18 median eyes ; 19 (above) lateral eyes ; 19 (below) nerve cord; 20 poison gland. 



dage (3rd leg). At their inner end they open into a vesicle. 

 The coxal glands are homologous with the green gland of 

 Malacostraca and the shell glands of the Entomostraca and 

 are derived from the coelom (see p. 317). 



The blood of scorpions contains haemocyanin, a blue-coloured 

 respiratory proteid, and the freshly shed blood is indigo in colour. 

 The corpuscles are oval and large. The heart is a dorsal tube 

 with seven chambers extending from the 7th to 13th segments. 

 A pair of valvular ostia open into the anterior end of each chamber 

 and a lateral artery leaves the posterior angle on each side. 

 Posteriorly the heart is continued throughout the metasoma 

 as a dorsal aorta. Anteriorly a truncus arteriosus (Fig. 522, 16) 



