64 COMPARATIVE ANATOMY. 



tions of the body, and preclude the possibility of untoward 

 pressure upon any one point. 



LYMPHATICS. 



The lymphatics are accessory to the sanguiferous system. 

 From the various tissues and organs they collect a fluid 

 (lymph) which is apparently a transuded portion of the liquor 

 sanguinis unappropriated by the tissues. In addition, they 

 derive nutritive material through the villi (lacteals) from the 

 food in the intestines. The liquids thus obtained are con- 

 veyed from smaller to larger vessels, until finally they empty 

 by one or more great trunks into the venous circulation. 

 (Leidy.) 



The lymphatics are confined to Yertebrata, and are con- 

 stant among its tissues ; exceptions; spleen of Chelonia, and 

 ganglia of nervous system. They are abundant without 

 valves or glands, as in Pisces; abundant with valves, but 

 without glands, as in Reptilia; less abundant, but possessing 

 both valves and glands, as in Aves and Mammalia. Pulsatile 

 vesicles are in some Yertebrata associated with the lymphat- 

 ics, as in Anguilla (eel), and Rana (frog). 



IX. 



EXCRETORY SYSTEM, 



THE function of removing from the organism the results of 

 waste of tissue is called excretion. No organ is specially 

 devoted to it among the Protozoa, Radiata, Mollusca below 

 Lamellibranchiata, and Articulata below Arachnida and In- 

 secta. Yarious bodies, apparently glandular, have had a 

 depurative office attributed to them, but oftener from exclu- 

 sion than possession of any certain knowledge of their nature. 

 With various Articulata, as in Insecta, many of the append- 

 ages to the alimentary canal particularly the conglobate 

 forms emptying their products near the oral or anal extrem- 

 ity are in part subservient to this purpose. The laoor of 



