CHAP.XXTX.] COMPARATIVE ANATOMY. 385 



Comparative Anatomy. In Entozoa, polyps, and medusae, no special respira- 

 tory organ exists. In star-fishes, and sea-urchins, among the echinodermata, 

 the sea water gains access to cavities among the viscera, and is renewed con- 

 tinually by special organs, principally cilia. The holothuria has an internal 

 system of arborescent tubes opening from the cloaca, receiving water, and, 

 according to Tiedemann, serving for respiration ; its branches end in vesicles. 

 In annelida, there are sometimes tufted branchiae, or gills, as in the arenicola, 

 or sand-worm, sometimes sacs opening separately, as in lumbrici and leeches. 

 The Crustacea have branchiae attached either to the feet or abdominal surface. 

 Of the arachnida, some, as the scorpion, have pulmonary sacs, or Lungs, with 

 parallel lamellae, situated on the abdomen in from one to four pairs, and each 

 opening by a separate stigma ; others have a system of ramified internal air- 

 tubes, termed tracheae, or spiral vessels (from a spiral thread in their wall) ; 

 and some both tracheae and pulmonary sacs. The myriapoda and all the 

 insecta have tracheae. These penetrate the internal organs to their remotest 

 parts, anastomosing freely, and open at several points on the surface. Insects 

 which breathe in water, as well as many aquatic larvae, have branchiae which 

 first separate air from the water, and then transmit it along the tracheae. The 

 respiration by tracheae is, probably, a very perfect one, the blood and tissues 

 being aerated throughout the body, at every spot in which they are being 

 deteriorated. 



Among Mollusca, some have Iranchioe, or gills, as the cephalopods, the 

 conchifera, and some gasteropods. Other gasteropods have a pulmonary sac, 

 or lung, e.g., the common snail. This sac opens and shuts so as to change the 

 air, and on its surface the venous blood is distributed ere it reaches the heart. 



Fishes present the greatest development of gills. There are four branchial 

 arches, bearing vascular plates with lateral offsets. Matteucci estimates the 

 surface of the gills of the common ray to measure 2,250 square inches. All 

 the blood is driven by the heart through the gills, to the aorta, and thus comes 

 into close proximity to the water, in contact with the branchial surface. The 

 capillary network has close and regular meshes. 



Reptiles have a rudimentary form of lung, combined, in many instances, with 

 gills, during a part of the whole of life, e.g., in the frog, the gills exist only in 

 the tadpole state : in the proteus, they remain through life. The pulmonary 

 sacs of reptiles are more or less cellulated on their inner surface, and receive 

 a portion only of the venous blood in each circuit. 



In Birds and Mammalia, respiration is much more active, being performed 

 by means of large and highly divided lungs, placed within a bony framework, 

 capable of receiving and rapidly renewing the air in large quantities, and giving 

 passage to the whole blood of the body on its way from the veins to the 

 arteries. In Birds, there is a series of openings from the pulmonary air-tubes, 

 by which the air gains access to passages and spaces among the other organs 

 and tissues, rendering the body specifically lighter, and, perhaps, in some 

 degree, aiding respiration. Further varieties in the structure of the lungs, 

 modifying their respiratory power, will be alluded to when the human lungs 

 have been described. 



Organs of Respiration in Man. The lungs, placed in the thoracic 

 cavity, receive air by the nasal passages and trachea, and venous 

 blood from the right side of the heart to transmit it to the left. 



