CIRCULATION. 199 



The more general resemblance between the arachnid and vertebrate circu- 

 lation is shown by the direction of the blood currents and by the distribution of 

 the main arterial and venous trunks. (Fig. 118.) In Limulus, the blood flows 

 laterally and neurally through five pairs of aortic trunks. The anterior pair 

 i.e. (internal carotids) go to the base of the brain, where they form a closed circle 

 around the oesophagus c.w. (circle of Willis around the infundibulum, Figs. 43, 

 44) and then backward along the brain and spinal cord. The following four 

 pairs are short trunks opening directly into two longitudinal channels in which 

 the blood flows forward ex.c. (external carotids) and backward (radices aortse) 

 into the unpaired aorta ao. Two large venous trunks (cardinals) collect the 

 blood from the anterior, lateral, and posterior parts of the body and conduct it to 

 the gills. 



Important changes, however, have taken place that we cannot explain satis- 

 factorily. In vertebrates, the ostia have evidently closed without leaving any 

 trace behind; and apparently one of the posterior pairs of the venous channels, 

 br.c. now opens directly into the posterior end of the heart, instead of into the 

 pericardial chamber (Cuvierian ducts). The relation of the gill chamber to the 

 aorta has also been radically changed. 



The curvature of the vertebrate heart, its splitting at the posterior end 

 (vitelline veins), and its elimination from the trunk segments are more readily 

 understood. These conditions are undoubtedly produced by the "yolk navel," 

 which is in turn produced by the increasing size of the yolk sphere; that is, the 

 cardiac ends of the lateral plates belonging to the branchial segments are forced 

 by the increasing size of the yolk sphere to reach the haemal surface of the egg in 

 the gradually shortening area between the overgrowing, precocious forebrain and 

 the anterior margin of the uncovered yolk surface (yolk navel). (Fig. 17.) As 

 this cardiac area is being continually shortened by the increasing precocity of the 

 forebrain, and by the increasing size of the yolk sphere, and as the heart itself is 

 meantime increasing in volume, it is forced to assume the S-shaped loops so 

 characteristic of vertebrates, in order to occupy the only space that is left open 

 for it. (Fig. 44.) These loops, once initiated, are accentuated by the unequal 

 mechanical stress of the enclosed blood current, which continues to sculpture and 

 mould the heart walls, as a river its banks, till organic equilibrium is again 

 reached in the four chambered heart of mammals. 



The splitting of the posterior end of the heart in vertebrate embryos is the 

 direct result of the increasing size of the yolk sphere, which favors the early 

 concrescence on the haemal side of the head of the anterior cardiomeres, but delavs 



j 



the concrescence of the more posterior ones, because they necessarily appear later 

 than the anterior ones, and have to travel over the arcs of larger circles. Thus 

 the ununited posterior cardiomeres may form two divergent, pulsating vessels 

 (vitelline veins) along the sides of the yolk navel, long after the anterior ones have 

 united to form a single tube. (Figs. 17, 23.) 



