PHYSIOLOGY OF THE INVERTEBRATA. 231 



sioDS. Ill the higher forms the entrance and exit of air is 

 regulated by valves placed at the external openings (stig- 

 mata) of the trachea3, and provided with muscles, by which 

 they can be shut." (Huxley.) 



In some of the lowest orders of this class there is no higher 

 form of respiration than that by the general surface of the 

 body. In the Acarina (represented by Acarus), Ara7icina 

 (represented by Epciva), and the Arthrogastra (represented 

 by Scorpio), we have simple tracheal respiration in the first- 

 mentioned order ; in the second, respiration is performed by 

 two stigmata opening into trachea}, and several others opening 

 into pulmonary sacs ; and in the third order, all the stigmata 

 open into pulmonary cavities or sacs. 



In Scorpio, which is the highest of the Arachnida, there 

 are no tracheal tubes, the animal breathing wholly by pul- 

 monary sacs. This rudimentary lung consists of a vascular 

 lining membrane extended into several folds, which are in 

 close relationship to the margins of the openings, and thus 

 afford an increase of surface for the contact of blood and 

 air. 



Tissue-respiration,* by means of pigments (myohtematin 

 and probably histohasmatin), also occurs in the Arachnida. 



The Crustacea. 



These animals breathe by means of branchia), which are 

 highly developed in the Crustacea. 



These organs contain true blood-vessels of a venous nature. 

 The carbonic anhydride from tissue-combustion passes out 

 into the water around, whilst the oxygen dissolved in the 

 water passes into the blood, "The access of fresh water to 

 the branchiae is secured by their attachment to some of the 

 limbs ; and in the higher Crustaceans, one of the appendages, 

 the second maxilla, serves as an accessory organ of respiration. 

 Although especially adapted for aquatic respiration, they 



* MacMunn, in Pl.ilos. Trans, of Rcyal Society, i8f6, pt. i, p. 272. 



