54 



ON CLASSIFICATION. 



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. 



Fig. 26. 



VI VII VIII IX X 



Fig. 26. — Diagrammatic section of a Scorpion, the locomotive members being cut away. 

 ", 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 chelicerse, or antennas. 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 following common characters of the Arachnida, how- 

 ever, help out our diagnosis in practice. They 'neverj possess 

 more than four pairs of locomotive limbs, and the somites of 

 the abdomen, even when the latter is well developed, are not 

 provided with limbs. Again, in the higher Arachnida (Fig. 26), 

 as in the higher Crustacea, the body is composed of twenty 

 somites, six of which are allotted to the head ; but, in the former 

 class, one of the two normal pairs of antennae is never developed, 

 and the eyes are always sessile, while, in the highest Crustacea, 

 the eyes are mounted upon moveable peduncles, and both pairs 

 of antennae are developed. 



