THE ARTHROGASTRA. 



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

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

 and has two valvular apertures, by which blood is udimtt, d 

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

 </i\ es 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 afferent pulmonary 

 sinuses, 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. SI. A, pulmonary sac. 5, respiratory leaflets of Scorpio occiianw. (After 



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- 

 on tos with the exterior by the stigma (Fig. 87). The orpin, 

 in fact, somewhat resembles a porte-monnaie with many pock- 

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

 exposed to the influence of the air, is carried by <-ft\--n ,,t pul- 

 monary sinuses to the pericardial sinus. Expiration is elV < t- 

 ed by muscles which pass vertically between the sterna and 

 terofa of the free somites. 



The bilobed cerebral ganglion supplies nerves to tho oyos 

 and ehelieeni?, and is connected by thick commissure with 

 the post-cesophageal ganglion, a la rue oval mass, whence 

 branches are given to the maxillae and following somites. A 

 long cord formed by two closely-applied commissures passes 



