268 PHYSIOLOGY OF THE INVERTEBRATA. 



The Arachnida. 



(a) In the Scorpion the posterior extremity of the abdomen 

 is armed with a sort of hooked claw, which, when the animal 

 is in motion, is always carried over the back in a most 

 threatening attitude. This claw-like organ is the sting, and 

 at its base are situated two poison-glands whose ducts pass 

 into the point of the sting, so that when the animal strikes 

 with its weapon, a small portion of the poison or vemon is 

 instilled into the wound. The sting is a weapon of offence. 



(&) In the Araneina the poison gland is lodged in the 

 cephalo-thorax, and the duct of it opens at the summit of 

 the terminal joint. It will be noticed that in the Araficina 

 the poison gland is situated in the anterior part of the 

 body, whereas in the Arthrogastra it is in the posterior 

 part. 



But " the most characteristic organ of the Araneina is the 

 arachnidium, or apparatus by which the fine silky threads 

 which constitute the web, a,re produced." 



In Epeira diadema this apparatus contains a thousand 

 glands with separate ducts. These ducts secrete the viscid 

 material which ultimately hardens into silk. " The glands 

 are divisible into five different kinds (aciniform, ampullate, 

 aggregate, tubuliform, and tuberous), and their ducts ulti- 

 mately enter the six prominent arachnidial mammillEe, which 

 in this species project from the hinder end of the abdomen. 

 The superior and inferior mammilla are three-jointed ; the 

 middle one is two-jointed. Their terminal faces are trun- 

 cated, forming an area beset with the minute arachnidial 

 papillae by which the secretion of the glands is poured 

 out." 



The Spider usually commences its thread by applying the 

 spinnerets to some fixed object ; to this the viscous secretion 

 attaches itself, when the movements of the animal are 

 sufficient to draw out the materials necessary for the con- 



