THE ARACHNIDA. 



55 



XXI. THE AKACHNIDA. 



It is this extreme elasticity, if I may so speak, of the 

 crustacean type which renders the construction of any defini- 

 tion of the Crustacea, which shall include all its members and 

 exclude the present class, so difficult. For the Spiders, 

 Scorpions, Mites, and Ticks, which constitute this class, possess 

 all the characters which have been just stated to be common to 



Fig. 28. 



XV 



XIV 



IV V VI VI1V1I1 IX X XI XII 



Fig. 28. Diagrammatic section of a Scorpion, the locomotive members being cut away, 

 a, Mouth leading into the pharyngeal pump. The large labrum lies above the mouth, 

 and at the side of it are the bases of the large chelae, or mandibles, iv., and above 

 them the chelicerae, or antenna?. VI. to xx. Somites of the body. T, Telson ; 6, intes- 

 tine ; c, anus ; d, indicates the position of the heart, e, the pulmonary sacs ; /, a 

 line indicating the position of the ganglionic chain ; g, the cerebral ganglia. 



the Crustacea save one ; when they are provided with distinct 

 respiratory organs, in fact, these are not external branchiae, 

 adapted for breathing aerated water, but are a sort of involution 

 of the integument in the form of tracheal tubes, or pulmonary 

 sacs, fitted for the breathing of air directly. But then many of 

 the lower Arachnida, like the lower Crustacea, are devoid of 

 special respiratory organs, and so the diagnostic character fails 

 to be of service. 



The following common characters of the Arachnida, how- 



