124 VERTEBRATA. 



which bathes the gills and serves for respiration to pass from the 

 pharynx into the branchial cavity. On the external side the gills 

 are often protected by a cutaneous fold or by an operculum, at the 

 lower or posterior margin of which there is a long slit for the 

 passage outwards of the water from the branchial cavity. The gills 

 may, however, project as uncovered external appendages (external 

 gills of Amphibians and Selachian embryos). 



In the lower Yertebrata lungs and gills may coexist in the same 

 animal, and in fishes the lungs are represented by a morphologically 

 equivalent organ the swimming bladder. Lungs, however, in their 

 more complete development are only found in the higher and for 

 the most part warm-blooded Vertebrates. In their simplest form 

 they appear as two sacs filled with air and opening by a common air 

 passage (trachea) into the pharynx. The walls of the pulmonary 

 sacs contain the respiratory capillaries ; their surface is usually 

 increased by folds and projections which give them the appearance 

 of a spongy organ or of an organ traversed by tubes. The two lungs 

 often extend far into the body cavity, but in the higher Vertebrates 

 they are confined to the anterior part of the latter which may be 

 more or less completely separated off from the hinder part of the 

 body cavity by a transverse partition the diaphragm and is then 

 called the thoracic cavity. 



Aerial respiration also requires a continual change of the medium 

 serving for respiration; the exchange of the used-up air saturated 

 with carbonic acid gas for the atmospheric air rich in oxygen. This 

 exchange is effected by various mechanical arrangements on which 

 the so-called respiratory movements are dependent. These move- 

 ments take place in all those Vertebrates which breathe by means of 

 lungs, but are most complete in the Mammalia, in which they con- 

 sist in alternating rhythmical contractions and dilatations of the 

 thorax. 



At the entrance of the trachea and in connection with the organs 

 of respiration is the vocal organ (larynx), which is usually formed by 

 a modification of the upper portion of the trachea. The larynx 

 contains vocal chords, and opens into the pharynx by a narrow slit 

 (glottis) which is usually capable of being closed by an epiglottis. 



The circulatory organs are in close relation with the respiratory 

 organs. The vascular system is always closed and contains red blood 

 (except in Amphioxus and the Leptocephalida, where the blood is 

 \vhite). The red colour of the blood, which was formerly held to be 

 the essential character of blood (Aristotle), is due to the presence of an 



