XTII * VASCULAR SYSTEM 331 
In 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! has shown that they open into the mid- 
eut 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 ning. 
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. Claparéde’s * 
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 les 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 Etudes sur la circulation du sang chez les Aranées du genre Lycose. Utrecht, 
1862. 
