GENERAL PRINCIPLES OF ZOOLOGY. 



a blood-vascular system as has here been described is called 



a closed one, because the blood always 

 flows in special tubes provided with 

 their own walls. Opposed to the closed 

 stands the lacunar blood-vascular system ; 

 here the blood-vessels lose, after a time, 

 the character of tubes and become wide 

 cavities, which, without special walls, 

 are enclosed between the intestines 

 and other organs. The ccelom serves 

 as such a cavity with especial fre- 

 quency. 



Example of Lacunar Blood-vascu- 

 lar System. The best example of a la- 

 cunar blood-vascular system is furnished 

 by the insects and "thousand-legs," 

 which have only the heart and short 



F.G. e 3 .-Anterior end of arterial trunks; from the ends of the 



stumps of the arteries the blood enters 

 the body-cavity, and from the body- 

 cavity through lateral slits again enters 



ries going from the heart, i t /T?- t: \ \\f\ it- 



emptying the blood into the heart (rig. 63). Within the groups 



of arthropods and molluscs are found all 

 grades between so extreme a case of a lacunar blood-vascu- 

 lar system and the almost completely closed one. Here 

 appears anew the closest correlation of the circulatory and 

 respirator}' organs, and indeed there exists a determining 

 influence for this. If the respiration is diffusely dis- 

 tributed over or through the body, and the distribution of 

 the oxygen goes on without special vessels, the circulatory 

 apparatus is very simple; on the other hand, if the respira- 

 tion is connected with definitely restricted areas, and a 

 regular distribution of oxygen is necessary, it is differen- 

 tiated into heart, arteries, veins, and capillaries. Upon 

 this point we may compare the conditions found in crus- 

 taceans, spiders, and insects. 



Lymph-vessels- Finally, a more special part of the 

 blood-vascular system is the lymph-vascular system, which 



