TYPE ARACHNID A. 451 



rectum connects with the anus situated at the posterior ex- 

 tremity of the body. 



In the genus Mygale and allied forms two pairs of stig- 

 mata are found near the anterior portion of the abdomen, 

 both of which lead into cavities containing lung-books. 

 In the majority of forms, however, but one pair of lung-books 

 (lb) occurs, the second pair of stigmata opening into a tracheal 

 tube (tr) extending into the cephalothorax and terminating in 

 a bunch of unbranched tracheae, a similar bunch arising near 

 its base and extending backwards into the abdomen (Seges- 

 tria). In some forms the second or tracheal stigmata may 

 be situated far back upon the abdomen, and may be united 

 to a single median transversely-elongated cleft, from which a 

 bunch of unbranched (Attus) or branched tracheae arises. 



The heart (ht), which lies in the abdomen, is enclosed 

 within a so-called pericardium and possesses three pairs of 

 ostia. It is continued anteriorly and posteriorly into aortse, 

 and gives off also lateral arteries, all of which open after rel- 

 atively short courses into the lacunar spaces. The blood is 

 returned to the pericardial cavity, whence it passes into the 

 heart, the greater portion on its way to the pericardium pass- 

 ing through the lung-books. 



The nervous system consists of a syncerebrum (ce) and a 

 la,rge cephalothoracic ganglionic mass (tg). In addition to 

 the nerves to the appendages, a posterior nerve arises from 

 this mass and passes backwards towards the abdomen, in 

 Mygale dilating at the junction of that region with the cephalo- 

 thorax into a pair of small ganglia. A sympathetic or visceral 

 system, consisting of a nerve arising by paired trunks from 

 the brain, is distributed to the anterior portion of the diges- 

 tive tract. The eyes are usually numerous, three or four pairs 

 occurring on the anterior portion of the cephalothorax, their 

 arrangement varying in different genera. 



Coxal glands have been found in several forms in connec- 

 tion with the third pair of legs, but have not been found to 

 open to the exterior in the adult. The reproductive organs 

 open in both sexes by a single opening situated near the 

 anterior end of the abdomen between the anterior stigmata. 

 The ovaries (ov) are paired, or may unite to form a ring, and 



