ARACHNIDA 517 



mentary appendages, the chilaria\ it is entirely missing in the adult 

 scorpions. In addition to this segment there is a maximum of twelve 

 segments and a terminal appendage, the telson, which is attained only 

 by the embryo scorpions and the eurypterids; the Palpigradi and 

 Pseudoscorpionidea have one less. In all these cases, there is a 

 differentiation of the segments into two regions, the meso- and meta- 

 soma. In Limuliis there are six segments only, but in the related 

 extinct genus, Hemiaspis, there are three more. The Solifugae show 

 ten. In the spiders, mites and phalangids, the body is much 

 shortened; the phalangids have the anterior segments united to the 

 prosoma. Lastly, the telson may be a sting in the scorpions, a jointed 

 sensory flagellum in the Palpigradi, a fin in some eurypterids or a 

 digging stick in others and in Limulus. 



A typical feature is the suctorial alimentary canal. The mouth is 

 usually narrow and situated just behind the chelicerae; only in 

 Limulus has it moved backwards, become enlarged and surrounded 

 by the basal joints (gnathobases) of all the prosomatic appendages ; in 

 the scorpions the appendages of the 2nd-4th segments form gnatho- 

 bases; the Palpigradi and Solifugae have no gnathobases. In all 

 arachnids, except Limulus^ the food is fluid and is drawn through a 

 narrow oesophagus into a sucking stomach and thence into a straight 

 mid gut, which is by far the longest part of the gut, and receives the 

 openings of the digestive coeca ; often, as in scorpions, there are several 

 of these, segmentally repeated, very much branched and forming a 

 compact " liver "-hke organ. There may be important salivary glands 

 entering the fore gut as in the scorpions. Posteriorly the mid gut, 

 except in Limulus, gives off Malpighian tubules. The hind gut is short. 



The respiratory organs of the Arachnida are distributed as follows, 

 (i) *'Gill books" in the aquatic form, Limulus, and probably in the 

 extinct eurypterids. (2) "Lung books" in the terrestrial scorpions 

 and Pedipalpi. (3) A combination of lung books and tracheae in the 

 spiders. (4) Tracheae alone in the Solifugae, Pseudoscorpionidea, 

 Phalangida and Acarina. (5) Lastly, in the Palpigradi, smaller acarines 

 and other forms, there are no special respiratory organs and exchange 

 of gases takes place through the skin. 



As the Arachnida apparently form a natural group, efforts have 

 been made to derive these various methods of respiration one from 

 the other. The gill books (Fig. 353) are stated to be the most primitive 

 respiratory organs. They are piles of leaflets, in which blood circulates, 

 attached in each segment to the posterior face of freely oscillating 

 plates, which are possibly appendages, resembling the abdominal 

 appendages of the Isopoda which are also respiratory in function. 

 There is a special muscular mechanism for opening and shutting the 

 leaflets in the water and thus facilitating gaseous exchange. In the 



