230 



ARTHROPODA 



[CH. 



organs. 



The blood takes the oxygen to the various tissues and 



takes from them the carbon 

 dioxide which is removed from 

 the body at the same centres. 

 In the Tracheata however the 

 air is itself conveyed by means 

 of the tracheae to all the cells 

 of the body and the gaseous 

 exchange takes place on the 

 spot. The blood has in the 

 Tracheata lost one of its chief 

 functions, the respiratory one, 

 and exists chiefly as a nutritive 

 fluid bathing the alimentary 

 canal and taking up from it the 

 soluble food which it conveys to 

 the other tissues. It is kept in 

 circulation by a contractile heart 

 which lies along the middle 

 dorsal line of the animal. In 

 Lithobius this heart has a pair 

 of ostia or openings in each 

 segment, into which the blood 

 from the pericardium pours, 

 only to be sent out of the heart 

 again at its anterior end into 

 the general cavity of the body, 

 for here the heart has an open- 

 ing and there is no system of 

 smaller vessels or capillaries. 



One of the peculiarities 

 associated with the above- 

 mentioned method of breathing 

 is the nature of the excretory 

 organs which rid the body of its 

 nitrogenous waste. In Peri-. 

 patus we find more or less 

 typical coelomiducts, and we 

 meet with modifications of these 



in the cox al glands of some Arachnids, but in the Myriapoda 

 and in the Insecta these organs are wanting and their place is 



Fio. 95. Lithobius forficatus, dissected 

 to show internal organs x about 2. 

 After Vogt and Yung. 



1. Antenna. 2. Poison claw. 



3. Salivary gland. 4. Walking legs. 

 5. Ventral nerve-cord. 6. Mal- 

 pighian tubule. 7. Vesicula semin- 

 alis. 8. Small accessory gland. 



9. Large accessory gland. 10. Un- 

 paired testis. 11. Alimentary canal. 



