2o8 THE HEART. 



of the experiments. Although he appears to have been influenced only by his 

 experiments on the living heart of Limulus, he has come to conclusions similar 

 to our own. He says {Am. Journ. Phys., 1905, p. 472) : 



"We find in the heart of Limulus a condition similar to that in the vertebrate 

 heart, the venous end of the heart exhibiting the greatest automatism, the aortic 

 end, the least, or no, automatism." There is another similarity between them, 

 in that "the regions of the heart exhibiting the greatest automatism have the 

 greatest number of ganghon cells." "And still another similarity in the dis- 

 tribution of the ganglion cells with reference to the myocard." " In the vertebrate 

 heart, they are situated, in the main, on the surface of the myocard, and this is also 

 their position in the Limulus heart." 



VII. The Heart. Summary and Conclusion. 



Interpreting the preceding data on the morphology, minute structure, and 

 the activities of the heart, we may draw the following conclusions : 



1. The heart of Limulus may be divided into two parts: a. An anterior, or 

 vagus division, consisting of three cardiomeres derived from the mesodermic 

 segments of the sixth, the seventh, and the eighth metameres, i.e., from the sixth 

 leg, chelarial, and opercular metameres. The segmental cardiacs of this division 

 are united with the pericardial nerves but not directly with the cardiac ganghon. 

 The vagus division of the heart has a greatly diminished ganglionic center and 

 is devoid of the giant ganglion cells. It exhibits little or no automatism, b. 

 The posterior, or branchial division of the heart is derived from the mesoderm 

 of the five gill-bearing metameres. Each of these cardiomeres is connected 

 with its corresponding neuromere by a segmental cardiac nerve, from which 

 branches go to the cardiac ganglion and to the pericardial nerves. It contains 

 the greater part of the small ganglion cells and all of the largest bipolar cells. 

 It exhibits marked automatism. In the dead heart there is a perceptible con- 

 striction between the vagal and the branchial divisions. 



2. The vagal and the branchial divisions of the heart in Limulus are compar- 

 able with the bulbar and ventricular divisions of the heart in \'ertebrates. That the 

 heart, as a whole, is comparable with that of the vertebrates is shown by the fact that 

 both organs arise on the haemal side of the head by the concrescence of mesoblastic 

 segments derived approximately from the same metameres. In both cases, the 

 absence of the cardiomeres in the forebrain region, and in the greater part of the 

 hindbrain region, may be traced to the absence there of the segmental lateral 

 plates of mesoderm. 



3. The giant bipolar cells of the branchial division of the heart of Limulus are 

 the primary agents in producing the rhythmic beat of the entire heart. Any 

 part of the heart separated from these cells ceases to beat. When the heart is 

 stimulated by laying the electrodes on the under side of the median nerve, the 

 only place where the giant nerve cells are fully exposed, the heart at once ceases 

 to beat. 



