GENERAL ORGANOLOGY 



103 



the arteries and veins of the systemic circulatory system; the dark blood 

 containing carbon dioxide is called venous, and the bright red, containing 

 oxygen arterial blood, since the former flows in the veins, the latter in the 

 arteries. These terms are entirely unsuitable, as can readily be seen 

 from the diagram (fig. 66), because they easily lead to the false assumption 

 that veins must always conduct blood containing carbon dioxide, and 

 arteries always oxygenated blood. On the contrary, the diagram shows 

 that, in the respiratory circulation, the conditions must be the reverse of 

 those in the systemic circulation, since here the arteries contain 'venous' 

 while the veins contain 'arterial,' blood. 



Closed and Lacunar Blood-vascular Systems. — Such a blood- 

 vascular system as has here been described is called a dosed 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 Ijlood- 

 vessels lose, after a time, the character of tubes and 

 become wide cavities, or sinuses, which, without 

 special walls, are enclosed between the intestines and 

 other organs (htemocoele, supra). 



Example of Lacunar Blood-vascular System. 

 — The best example of a lacunar blood-vascular 

 system is furnished by the insects and myriapods, 

 which have only the heart and short arterial trunks ; 

 from the ends or the arteries the blood enters the 

 hasmocoele, and from this through lateral slits (oslia) 

 again enters the heart (fig. 67). In the arthropods 

 and molluscs are found all transitions between so 

 extreme a case of a lacunar system and the almost 

 completely closed one. Here appears again a close 

 correlation of the circulatory and respiratory organs, 

 the latter determining the development of the 

 former. If the respiration be diffusely distributed 

 over or through the body, the circulatory apparatus 

 is very simple; on the other hand, if die respiration 

 be connected with definitely restricted areas, the 

 apparatus is differentiated into heart, arteries, veins, 

 and capillaries. Details may be found in the sections on crustaceans, 

 spiders, and insects, infra. 



Lymph-vessels. — A special part of the vascular system is the lymph 

 system, known only in vertebrates. In the capillary region of the body 

 proteids may pass into the tissues, but it is e^ddent that an overflow 



Fig. 67. — .Vnterior 

 end of the heart of 

 Scolopendra (after 

 Newport), ac, cepha- 

 lic artery; ah, arterial 

 arcli ; al, lateral artery ; 

 fm, alar}' inuscles (ahe 

 cordis); ht:, chambers 

 of heart; 0, ostia. 



