CIRCULATION. 199 



The more general resemblance between the arachnid and vertebrate circu- 

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



main arterial and venous trunks. (Fig. ii8.) In Limulus, the blood flows 

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



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

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



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

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



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

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

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

 : gills. 



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

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

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

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

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

 ■ta has also been radically changed. 



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

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

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

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

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

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

 : gradually shortening area between the overgrowing, precocious forebrain and 

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

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

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

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

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



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

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

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

 ched in the four chambered heart of mammals. 



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

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

 icrescence on the haemal side of the head of the anterior cardiomeres, but delays 



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

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



ununited posterior cardiomeres may form two divergent, pulsating vessels 

 :elline veins) along the sides of the yolk navel, long after the anterior ones have 

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



