CIRCULATORY SYSTEM IN ARACHNIDA 177 



and the position of organs in front of the mouth, which morpho- 

 logically have to be considered as postoral. Notice the plastro- 

 buccal muscles going ''from the anterior neural side of the plastron 

 to the oesophagus" and the strands of muscles attaching the 

 proventriculus to the carapace in the region of the median eyes. 

 The former undoubtedly represent the pharyngeal dilators of 

 Archnida, the latter the dilators of the sucking-stomach in spiders 

 and the corresponding pair of dorsoventral muscles in the scor- 

 pion. Although considerably in front of the posterior edge of 

 the carapace in Limulus, these muscles are not far in front of 

 the aortic valve. Notice that in severing the carapace from 

 the abdomen with a knife, the opercular plate remains with the 

 carapace. Notice, further, that the suboesophageal ganglionic 

 mass in Limulus consists of seven neuromeres, the sixth be- 

 longing to the chilaria and the seventh to the opercula; that, 

 owing to the perihaemal type of blood system, the vessels for 

 the chilaria and opercula issue from the vascular ring; notice 

 all this and you get the idea of what happened to Limulus in the 

 course of its phylogenetic development. On the ventral surface 

 two somites, corresponding to the first and second abdominal 

 somites in Arachnida and characterized by the chilaria and oper- 

 cula, became fused with the thoracic somites, while at the same 

 time the corresponding neuromeres moved forward and fused 

 with the suboesophageal ganglionic mass. On the dorsal surface 

 a general displacement forward took place. In this displacement 

 two things remained unchanged: the position of the mouth and 

 the attachment points of the foregut and of the heart in the re- 

 gion of the aortic valve. What was above and behind the mouth, 

 with the forward bending of the back came to lie in front and 

 above the mouth. Part of the heart followed the displacement 

 because of the permanent attachment at the aortic valve. Of 

 the tergites, those of the chilarial and opercular somites had to 

 follow the forward motion of the original carapace and were drawn 

 into the hollow of the horseshoe-shaped carapace as it was formed 

 through the forward displacement. These tergites fused with the 

 carapace along their front and sides, but are still visible even in 

 the adult and especially in the so-called trilobite stage of the 



