ORGANS OF RESPIRATION. 535 



tubes, and the two anterior into circumscribed pulmonary 

 cells. 



Of the pulmonated forms, some, as the scorpion, have eight 

 distinct internal respiratory sacs, opening by separate stig- 

 mata ; others, as mygale, have four sacs, and the most con- 

 centrated form of these organs is that of the spiders, 

 which have only two symmetrical pulmonary cells. On 

 the ventral surface of the abdominal segments of the scor- 

 pion, four pairs of elongated oblique stigmata open into eight 

 broad short quadrangular sacs, each of which is divided into 

 nearly twenty small flat cells, by internal parallel septa, 

 giving a pectinated appearance externally to the shut margin 

 of the sac, which may be compared to the incipient develop- 

 ment of peripheral cells in the lungs of reptiles. Four simi- 

 lar sacs, also with soft white vascular parietes, open, by 

 four stigmata, on the ventral surface of the abdomen in the 

 mygale, and one similarly divided pulmonary sac opens, in 

 spiders, by a transversely elongated stigma on the anterior 

 part of the ventral surface of the abdomen. There is thus, 

 in passing through the tracheated, the pneumo-tracheated, and 

 the pulmonated forms of arachnida, a gradual transition from 

 the highly ramified and extended condition of the respira- 

 tory organs of insects, to the more concentrated and laminated 

 structure presented by the branchiae of Crustacea. 



As the blood is most extensively circulated through the 

 body in crustaceans animals, and their respiration is only 

 aquatic, their branchiae present an extensive surface, and 

 consequently a laminated structure, to receive a larger portion 

 of venous blood in capillary vessels, for aeration. The gills 

 are here generally attached to the bases of the articulated 

 feet, whether these lateral appendices of the segments be 

 masticatory, ambulatory, natatory, ovigerous, or simply res- 

 piratory ; and they receive the venous blood of the system 

 before returning to the heart, for distribution through the 

 body. In the lower orders of Crustacea, the branchiae are 

 commonly confined to the appendices of the segments near 

 the caudal end of the trunk, where they hang free in the sur- 

 rounding element, and are rapidly moved to and fro by the 

 organs which support them, as by vibratile cilia. But in the 

 higher species, they are attached to the ambulatory and mas- 

 ticatory appendices near the cephalic extremity of the body, 



