CIRCULATORY SYSTEM IN ARACHNIDA 163 



Although the diverticula are divided by the dorsoventral 

 muscles into ten groups, there are only six pairs of ducts con- 

 necting them with the midgut proper. The first pair of these 

 transverse ducts is in the cephalothorax and connects the two 

 cephalothoracic diverticula with the anterior portion of the mid- 

 gut. The first, second, third, and fourth abdominal diverticula 

 have a pair of transverse ducts each in the corresponding seg- 

 ments. The last pair of connecting ducts opens into the midgut 

 in the fifth abdominal segment. These ducts are long, extend- 

 ing backward over three segments and establishing a connection 

 between the midgut and all four posterior metameric groups of 

 diverticula. 



Transverse sections through the second to fourth abdominal 

 segments show that each pair of diverticula is composed of 

 two dorsal, two ventral, and one lateral lobes. All lobes are 

 filled with embryonic yolk and all those on the same side are 

 naturally connected with each other near their base. The dor- 

 sal right and left lobes are separated by the pair of dorsoventral 

 muscles. Below the midgut proper, which occupies approxi- 

 mately the center of a transverse section, the dorsoventral mus- 

 cles pass on the outside of the inner pair of ventral lobes. In 

 the cephalothorax only dorsal lobes are present. The last pair 

 of transverse ducts gives off side branches to the fifth and then 

 to the SLxth abdominal diverticula, while the ends of the ducts 

 lead into the seventh pair into which the eighth pair also opens. 



The hindgut begins in the terminal region of the fifth abdominal 

 segment. It is considerably smaller than the midgut, but has 

 both relatively and absolutely much thicker walls. It is devoid 

 of embryonic yolk and in the last postabdominal segment forms 

 a considerable widening beyond which it is again suddenly con- 

 stricted and opens with a small anus in the midventral line at 

 the end of the segment. 



The heart lies entirely in the preabdomen, extending from 

 the diaphragm which separates the latter from the cephalothorax, 

 almost to the end of the seventh abdominal tergite. Its an- 

 terior and posterior limits are clearly defined by the valves 

 of which I shall speak later. • The heart has seven pairs of ostia 



