MORPHOLOGY OF HIGHER TYPES 



125 



ventricle the blood is discharged into the dorsal aorta whence it passes 

 out to the systemic circulation. Upon its return it passes through the 

 right side of the heart to the lungs, and so on around its course. The 

 circulation is thus a double one. The pulmonary and systemic circula- 

 tions are kept absolutely distinct because the right and left chambers of 

 the heart are completely separated from one another. In animals with a 

 three-chambered heart, as in the frog, this distinctness does not prevail, 



for while there are two auricles there 

 is but a single ventricle. There is 

 therefore some mixing of blood in the 

 ventricle; but the structure of the 

 ventricle with its deep recesses and 

 the operation of valves in the principal 

 artery are such that the mixing of 

 venous and arterial blood is partially 

 prevented. 



Lymphatic System. — In addition 

 to the system of vessels carrying blood 

 in vertebrates there is a system which 

 carries lymph and which is called from 

 this fact the lymphatic system. It is 

 composed of minute spaces between 



Fig. 90.— Portion of a protonephridial 

 system from the tapeworm Taenia crassi- 

 collis. f, flame cell ; n, nucleus of excretory 

 tubule ; tu, excretory tubule. {From Hesse 

 and Doflein after Bugge.) 



Fig. 91. — Flame cell of a protonephridium 

 of a flatworm. ci, cilia within funnel shaped 

 cavity of flame ceil; n, nucleus. (From Hesse 

 and Doflein after Lang.) 



tissue cells which lead into thin-walled tubes, the lymph vessels. At in- 

 tervals, connected with the lymph vessels, in some vertebrates, are con- 

 tractile organs {lymph hearts) by whose contraction the lymph is forced 

 along. The lymph vessels empty their contents into the venous system. 

 In mammals they regularly connect with the left jugular vein. 



Excretory System. — The simplest form of excretory system occurs 

 in the rotifers, flatworms and certain other low invertebrates, and consists 



