An A rack n id. 119 



the external opening of the sexual organs, in the female 

 highly elaborated. The spinnerets will be found at the pos- 

 terior end of the abdomen as three pairs of papillae. They 

 are jointed, and probably represent specialized abdominal 

 appendages. The smaller middle pair of spinnerets are 

 covered by the much larger anterior and posterior pair, 

 while the anal papilla is closely folded against the latter 

 as a transverse chitinous elevation. If the anal papilla is 

 pressed back, the opening of the anus will be found. 



A small pore, or spiracle, lying in the median line, imme- 

 diately in front of the spinnerets, leads from a series of 

 branched abdominal trachece. This pore is often difficult to 

 find. It lies just in front of a thorn-like process which rests 

 just between the first pair of spinnerets. If the integument 

 at this point be carefully picked away, the tracheae will be 

 found. 



Make a drawing of the spider as seen from above, and 

 another as seen from below. 



Internal Anatomy. - - Carefully remove the integument 

 from the upper half of the thorax and abdomen. 



The Circulatory System. - -The heart is an elongated, 

 thin-walled tube which extends along the dorso-median por- 

 tion of the abdomen and is continued anteriorly, as the ante- 

 rior aorta, through the peduncle, into the cephalothorax. 

 The heart is really enclosed in a very delicate pericardium, 

 which holds the blood as it is returned from the lacunce of 

 the body, finally pouring it into the heart through certain 

 lateral ostia. The blood, leaving the heart through the ante- 

 rior and posterior aortas, is conveyed in arteries for only a 

 short distance. It soon enters lacunae and sinuses, in which 

 it pursues a definite course until it gathers in a large reservoir 



