56 



RESPIRATORY MECHANISMS 



the Janellidce, a small group of naked tropical snails, reaching 

 a maximal length of about 4 cm, in the isopod Porceliio, and 

 in the chilopod Scutigera. It provides a large respiring surface 

 taking up only a small space in the animal. A more detailed 

 description can be given for Scutigera. 



This animal has 7 dorsal spiracles each leading into a ves- 

 tibulum from which about 600 short tracheae arise (Haase, 

 1885, Fig. 30). In an animal weighing about 0.3 g, Krogh 



Fig. 30. Spiracle and tracheal lung of Scutigera. Ca. 60/1. (Haase.) 



(1920) measured the dimensions of these lungs and calculated 

 the oxygen pressure difference necessary to supply the meta- 

 bolic requirements. This worked out at about 1 mm of 

 mercury, showing that diffusion is ample, even if the spiracles 

 are narrow enough to reduce considerably the diffusion rate. 

 No respiratory movements appear possible. 



Typical diffusion lungs are the "book lungs" of the lung- 

 breathing arachnids (Fig. 31). Plateau (1887) studied scor- 

 pions and several species of Aranece and was unable to detect 

 any respiratory movements. The scorpions have 4 pairs of 

 lungs (Fig. 32) situated in the third to sixth abdominal seg- 

 ments and receiving venous blood from the ventral sinus. 

 Certain spiders (Tetrapneumones) have two pairs and others 

 (Dipneumones) are described as having only one pair, but in 



