n 4 THE HUMAN SPECIES 



undoubtedly serve as breathing organs, and the branchiae of 

 the annelida and the lamelliform extension of the integument 

 of the hirudinea must be unhesitatingly recognised as special 

 organs of respiration. 



So far, all these breathing organs have been purely external. 

 Internal organs of the nature of gills are first found in the 

 enteropneusta, and the tunicata possess an internal breathing 

 pouch, forming in its highest stage of development a distinct 

 apparatus attached to a special part of the pouch. In the 

 echinodermata respiration is carried on inside the ciliated 

 cavity of the body by means of a system of water vessels ; 

 similarly in the Asteroidea, but with the modification that csecal 

 tubes open radially from the alimentary canal, and, in the holo- 

 thurians, in the form of ramifying, tree-like organs. In each 

 case the organs are designed to facilitate the exchange of gases 

 by protrusion of the abdominal cavity, whereby water may be 

 poured over the external as well as the internal surface. 



Remarkable conditions prevail among the arthropoda : in 

 some members of the class (crustaceans) parts of the integu- 

 ment have been transformed into gills attached to the feet, 

 some are provided with a gill-cavity closed from the side or, 

 again, with a mantle which acts as a breathing organ uncon- 

 nected with the limbs ; others, such as the arachnida, myria- 

 poda and insects, possess a system of ramifying tracheal tubes 

 which either open outwardly at certain points and thus imbibe 

 air, or, as is the case among the larvae of many water-insects, 

 the tubes are externally closed. The molluscs are also 

 furnished with gills, whether in the form of a respiratory 

 mantle or, as in the brachiopoda, respiratory arms, in both 

 cases combined with the development of abundant cilia. Other 

 cases are known where the breathing cavity has arisen from 

 the fact of the gills being imbedded in the mantle, and, finally, 

 in the air-breathing pulmonata, the cavity, or a separated part 

 of the same, takes the form of a lung. 



Common to all the vertebrates is the possession of breathing 

 organs which originated in the wall of the intestinal tube and 

 are connected therewith. Here, too, a gradual development 

 may be traced from lower to higher forms. At the very base 

 of the scale stands the method of breathing by gills, founded 



