THE AR IHROGASTRA. 325 



line of the tergal aspect, between the eye-bearing shield and 

 the tail ; each chamber is wider behind and narrower in front, 

 and has two valvular apertures, by which blood is admitted 

 from the pericardial sinus at its postero-lateral angles. It 

 gives off small lateral arteries, and ends in front and behind 

 in a wide aortic trunk. Of these the anterior is larger than 

 the oesophagus, and both aortae give off branches which are 

 distributed widely through the body. A large trunk lies on 

 the tergal aspect of the ganglionic chain, and is united with 

 the anterior dorsal aorta, by a lateral aortic arch, on each side 

 of the body. The veins, on the other hand, are irregular pas- 

 sages, the blood of which is carried to two ajferent pulmonary 

 smuses, one for each set of respiratory organs. 



These respiratory organs are four pairs of flattened sacs, 

 which open externally by as many stigmata, on the sterna of 

 the four posterior free thoracic somites (Fig. 85, xi-xiv) in 



Fig. 87.—^, pulmonary sac. 5, respiratory leaflets of Scorpio occitanus. (After 



"Rl Q n f>l 1 a rrl "4 



Blanchard.) 



front of the tail. Each lies with one flat side sternal and the 

 other tergal, in front of its stigma, and its walls are so folded 

 as to divide its cavity into a multitude of subdivisions, each 

 of which opens into the common chamber which communi- 

 cates with the exterior by the stigma (Fig. 87). The organ, 

 in fact, somewhat resembles a porte-m.onnaie with many pock- 

 ets. The blood circulates in the folds, and, after being thus 

 exposed to the influence of the air, is carried by efferent pul- 

 inonary sinuses to the pericardial sinus. Expiration is effect- 

 ed by muscles which pass vertically between the sterna and 

 terga of the free somites. 



The bilobed cerebral ganglion supplies nerves to the eyes 

 and chelicerae, and is connected by thick commissures with 

 the post-oesophageal ganglion, a laro-e oval mass, whence 

 branches are given to the maxillae and following somites. A 

 long cord formed by two closely-applied commissures passes 



