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GENERAL PRINCIPLES OF ZOOLOGY. 



Example of Lacunar Blood-vascular System. — The best exam- 

 ple of a lacunar blooil-vasculav system is furnished by the insects 

 and myriapods, which have only the heart and 

 short arterial trunks; from tlie ends of the 

 arteries the blood enters the hajmoccele, and 

 from this through lateral slits (ostia) again 

 enters the heart (fig. G6). In the groups of 

 arthropods aiid molluscs arc found all transi- 

 tions between so extreme a case of a lacunar 

 blood-vascular system and the almost com- 

 pletely closed one. Here appears again a 

 close correlation of tlie circulatory and respir- 

 atory organs, the latter determining the 

 development of the former. If the respira- 

 tion be diffusely distributed over or through 

 the body, and the distribution of the oxygen 

 goes on without special vessels, the circula- 

 tory apparatus is very simple; on the other 

 liand, if the respiration be connected with 

 definitely restricted areas, and a regular dis- 

 tribution of oxygen be necessary, the appara- 

 tus is differentiated into heart, arteries, veins, 

 iary and capillaries. Details may be found in the 



Firi. Hti.— Antei'ior end of 

 the heart of Scohipcii- 

 (Ira. (After Newport.) 

 Of. cephalic artery; ah. 

 arterial arch; al, later 

 al artery ; hn 



muscles ^f^^ ^f^^^^^] sections ou crustaceans, spiders, and insects. 



/lA", chain 

 f. ostia. 



intra. 



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

 himpti syston, which is known only in vertebrates. In the capil- 

 lary region of the body, it is true, jiroteids may pass over into the 

 tissues, but it is evident that a jiossible overflow cannot re-enter 

 the blood-vessels in the same way, on account of the higher pres- 

 sure prevailing in the capillaries. This overflow is conducted back 

 to the veins through the lymph-vessels. The lymph-vessels begin 

 with lacuniB in the tissues, and gradually pass into vessels with 

 defluite walls. The lymiih-vessels of the digestive tract are par- 

 ticularly important since, during digestion, they become filled Avitli 

 tlie jiroteid and fatty constituents of the digested food; they are 

 called the cJiylc-ressels, because they contain the chyle, distin- 

 guislieil from ordinary lymph by its milky color. 



Cold- and Warm-blooded Animals. — In connexion with the 

 blood-vascular system, two expressions arc much used but not 

 g(!nerally correctly umlerstood by tlie general public, viz., cold- 

 blooded and Avarm-liliioded — or, niort> corrcctlv, animals witli 



