CHAPTER LIIL 



THE CIRCULATORY SYSTEM 



To understand the modern interpretation of the circulatory system 

 it is necessary to have clearly in mind what is called the probable an- 

 cestral condition of this system in the lower forms of animals. Thus 

 one may observe how in each of the succeeding higher forms, something 

 is added to the development of the animal of the next succeeding scale 

 below. 



Some have thought that the original circulation consisted of a 

 lymphoidal liquid alone, and as time went by this type of circulation 

 specialized into what we now term a blood circulation. It is thus sup- 

 posed that the lymph vessels, as we find them in modern forms of living 

 animals, furnish a clue as to how the primitive systems of vessels ap- 

 peared. It is all quite speculative, however. Another explanation, which 

 has more plausibility in its favor, is that the main blood vessels are the 

 remnants of the segmentation cavity which has become obliterated by 

 the growth of mesoderm, the part not obliterated then becoming the 

 blood vessels. 



In any explanation that is built upon the Haeckelian "law" of bio- 

 genesis there not only remains much to be explained, but various occur- 

 rences even must be explained away. In this theory it is supposed that 

 much of the race history has been lost in development, while a develop- 

 ment of additional vessels of various kinds have covered up some of the 

 older developmental processes. 



Many blood vessels which should arise as fissures between other 

 tissues are found to be formed as solid cords of cells. These may later 

 form a lumen and be converted into tubes, or in other instances, vessels 

 which originate separately in the embryo, may fuse together during de- 

 velopment to form a single one. 



There are various main points, however, which must be understood 

 in any discussion of the blood system (Fig. 444, A, B). There is a dor- 

 sal tube carrying the blood toward the tail, from which various vessels 

 extend toward the right and left at almost right angles through the dor- 

 sal tube. Those that pass toward the outer side of the animal are called 

 somatic, those that pass toward the inner region are called splanchnic. 

 These transverse vessels connect with two ventral, longitudinal tubes, 

 one of which is in the wall of the digestive tract which runs headward 

 and unites with the other one which has passed through the ventral body 

 wall, so that, after the union of the two, a single tube is found coursing 

 to the head end of the body. In one of the lowest forms of chordates, 

 namely, Amphioxus, various parts of this system develop muscle walls 

 and then act as pumping organs. 



