1 68 MORPHOLOGY OF INVERTEBRATE TYPES 



and comparable to the leaves of a book. The air circulates be- 

 tween the lamellae, while the blood is oxidized inside of the 

 lamellae. The system of tracheal tubes is quite separate from the 

 lung-books. The single tracheal spiracle is situated on the ven- 

 tral surface of the abdomen, in front of the spinnerets. The 

 common stem splits very soon into two branched tubes pene- 

 trating among the cells of the various organs. 



Nervous system and sense organs. The central nervous 

 system of Agelena is characterized by a great concentration of 

 ganglia. The brain or supracesophageal mass is usually supposed 

 to consist of only two neuromeres, the protocerebron and the 

 deuterocerebron. But it is much more plausible that at least 

 four neuromeres compose the brain. The first neuromere is 

 represented by the first optic ganglia and supplies nerves to the 

 four lateral and the posterior median eyes. The second neuro- 

 mere is formed by the second optic ganglia and supplies nerves 

 to the anterior median eyes. The third neuromere apparently 

 disappears during development, while the fourth neuromere 

 supplies nerves to the chelicera. The brain is united to the 

 subcesophageal mass by short connectives forming a ring for the 

 passage of the oesophagus. The subcesophageal mass is situated 

 immediately above the sternum and below the endoskeleton, 

 so that a puncture of the sternum involves usually also a punc- 

 ture of the subcesophageal mass and causes paralysis of the 

 animal. The mass is composed of the last cephalic ganglion, 

 four thoracic ganglia, and at least three abdominal ganglia 

 (four abdominal ganglia in the scorpion). It supplies nerves to 

 the under lip, pedipalpi and legs. Posteriorly the mass is drawn 

 out into a long stem ending in another ganglionic mass in the 

 pedicel. This latter mass is quite small compared with the 

 thoracic mass and represents the fused and greatly reduced 

 remaining ganglia of the abdomen. 



The best developed sense in Agelena is that of touch. Every 

 hair, bristle and spine is supplied with a nerve fibre, with the 

 result that the slightest touch or the gentlest vibration of the 



