16 Mr. E. I. Pocock on the 



So far as internal organization is concerned, it is interest- 

 ing to note that the great development of the abdomen in the 

 Scorpions is correlated with a serially gangliated nervous 

 chord, a many-chambered heart, and four pairs of breathing- 

 organs. In all the other Arachnida in which the abdomen is 

 shortened, and its muscularity diminished, the nervous chord 

 is simplified by the disappearance of the ganglia, the cham- 

 bers of the heart are reduced in number, and the two posterior 

 breathing-organs atrophy. 



It is evident that in the Scorpions the posterior region of 

 the abdomen is the seat of great muscular activity. To repair 

 the muscular tissue and to absorb its waste products a rich 

 supply of blood is required, and the oxygenation of this blood 

 will be more efiiciently performed by four pairs of lung-sacs 

 than by fewer. But in all other Arachnida the abdomen is 

 little more than a vehicle for carrying generative and alimen- 

 tary glands, so that, seeing that in the Scorpions four pairs 

 of lungs are sufficient, it is not surprising that the rest of the 

 Arachnida have been able to dispense with two pairs. 



Again, with regard to the position of the apertures of the 

 breathing-organs, it has already been pointed out that in 

 Thehjplionus and Phrymis, which retain the so-called lung- 

 books, these apertures lie in front of sterna of the somites con- 

 taining the lungs. A beneficial result of this arrangement is 

 that the sterna in front of the apertures form movable oper- 

 cula to them, so that they can be opened and closed at will. 

 In the Aranese, where the terga and sterna mostly atrophy, 

 we find the remnants of these sterna retained as the opercula. 

 In the higher Araneae (Dipneumones) the posterior pair of 

 pulmonary sacs are replaced by tracheal tubes, the apertures 

 of which in most forms take advantage of the continuity of 

 the integument of the lower surface of the abdomen to migrate 

 to its hinder end in the wake of the spinning-appendages. 



In the Solifuga>, the apertures of the abdominal tracheai are 

 very small. In most forms, e. g. Solpuga, they are visible 

 on the surface of the sterna, but in Oaleodes they have 

 moved posteriorly until they lie behind the sterna and are 

 thus capable of being closed. This, as well as the small size 

 of the stigmata, must be an advantage to an animal living in 

 desert countries, where sand would be liable to block the 

 breathing-tubes. 



Another interesting point in connexion with these animals 

 is that a second pair of breathing-organs occurs on the ventral 

 surface of the cephalothorax behind the coxas of the fourth 

 appendages, as if to compensate for the small size of the 

 abdominal stigmata. The apertures of these organs must 



