VASCULAR SYSTEM 331 



Iii the abdomen it is surrounded by a large gland, the so- 

 called liver, and is dilated at one spot (Fig. 186) to receive the 

 ducts from this gland. The fluid elaborated by this large 

 abdominal gland has been shown to have more affinity with 

 pancreatic juice than with bile. 



The Proctodaeum consists of a short rectum, from the dorsal 

 side of which protrudes a large sac, the " stercoral pocket." At 

 its origin, the rectum receives the openings of two lateral tubes 

 which reach it after ramifying in the substance of the liver. 

 These have been called " Malpighian tubules," but their function 

 is unknown. Loman l has shown that they open into the mid- 

 gut and not into the rectum, and there is reason to believe that 

 true Malpighian tubules homologous to those of Insecta are 

 absent in Arachnida, where their place seems to be taken by the 

 coxal glands, which are considered to be the true excretory 

 organs. In most spiders they open near the third coxae. Like 

 the stomodaeum, the proctodaeum has a chitinous lining. 



Vascular System. The earlier investigations on the circula- 

 tion of the blood in Spiders were made by direct observations of 

 the movements of the blood corpuscles through the more or less 

 transparent integuments of the newly hatched young. Claparede's 2 

 results were arrived at by this method. It is invaluable for 

 demonstrating roughly the course taken by the blood, but in these 

 immature spiders the blood-system has not attained its full com- 

 plexity, and other methods of research have shown the spider to 

 possess a much more elaborate vascular system than was at first 

 suspected. 



The tubular heart lies along the middle line in the anterior 

 two-thirds of the abdomen, sometimes close up against the dorsal 

 wall, but occasionally at some little distance from it, buried in 

 the substance of the liver. It is a muscular tube with three 

 pairs of lateral openings or " ostia," each furnished with a simple 

 valve which allows the entrance, but prevents the exit, of the 

 blood. It is contained in a bag, the " pericardium," into which 

 the ostia open. Both heart and pericardium are kept in place 

 by a complicated system of connective tissue strands, by which 

 they are anchored to the dorsal wall of the abdomen. Eight 



1 Tijdschr. v. d. Nederl. Dierkundige Ver. (2), i., 1885-1887, p. 109. 



2 fitudes sur la circulation du sang chez les Aranees du genre Lycose. Utrecht, 

 1862. 



