230 PHYSIOLOGY OF THE INVERTEBRATA. 



Eespiration in tlie aquatic larvEe of certain insects is 

 performed by means of tracheal gills or branchiae. These 

 branchiae are delicate folds of the integument, and are richly- 

 supplied with minute tracheae. The oxygen dissolved in 

 water, wherein or whereon these larvae flit to and fro, passes 

 into the tracheae. This phase of respiration among the 

 Insecta is suggestive of the branchite of certain forms of the 

 Annelida, into which the vessels of the pseudo-haemal systems 

 enter. 



The larvae of lAhelhda and ySscJma present yet another 

 form of respiratory organ. " Although they possess a pair 

 of thoracic stigmata, these appear to have little or no 

 functional importance, but respiration is effected by pumping 

 water into and out of the rectum. The walls of the latter 

 are produced into six double series of lamellae, in the interior 

 of which trachese are abundantly distributed, and which play 

 the same part as the tracheal branchi^ just mentioned. 

 These rectal respiratory organs, in fact, appear to be a com- 

 plicated form of the so-called ' rectal glands ' which are so 

 generally met with in insects." 



Besides the systems of tracheal tubes, tissue-respiration is 

 well marked in the Insecta, for MacMunn has met with 

 myohaematin in abundance in these animals ; and it is 

 probable that histohaematin is also present, and both of these 

 pigments have a respiratory function. 



The Aeachnida.^ 



In these animals " trachese may exist alone, or be accom- 

 panied by folded pulmonary sacs, or the latter may exist 

 alone, as in the Scorpion. In this case these lungs* are 

 supplied by blood which is returning from the heart." 



"The flow of air into and out of the air cavities is 

 governed by the contractions of muscles of the body, 

 disposed so as to alter its vertical and longitudinal dimen- 



* See also Maeleod's paper in Bull. Acad. Boy. Scien. Belg., vol. 3, p. 779. 



