114 



GENERAL PRINCIPLES OF ZOOLOGY. 



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

 ple of a lacnnar blood-vascular system is furnished by the insects 

 and myriapods, which have only the heart and 

 short arterial trunks; from the ends of the 

 arteries the blood enters the haemocoele, and 

 from this through lateral slits (ostia) again 

 enters the heart (fig. 66). In the groups of 

 arthropods and molluscs are 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 the 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 

 hand, if the respiration be connected with 

 definitely restricted areas, and a regular dis- 



i. uu. .fi.ii bei-iui: eiiu \JL , ., . . " ,, , ., 



the heart of Scoiopen- tribution of oxygen be necessary, the appara- 

 tus is differentiated into heart, arteries, veins, 

 n!' alary and capillaries. Details may be found in the 

 f heart j sections on crustaceans, spiders, and insects, 



' 



FIG. 66. Anterior end of 



o, ostia. f 



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

 lymph system, which is known only in vertebrates. In the capil- 

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

 tissues, but it is evident that a possible 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 lacunae in the tissues, and gradually pass into vessels with 

 definite walls. The lymph-vessels of the digestive tract are par- 

 ticularly important since, during digestion, they become filled with 

 the proteid and fatty constituents of the digested food; they are 

 called the chyle-vessels, because they contain the chyle, distin- 

 guished from ordinary lymph by its milky color. 



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

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

 generally correctly understood by the general public, viz., cold- 

 blooded and warm-blooded or, more correctly, animals with 



