228 



Tlir INTI.RNAL ENVIRONMI-NT OF THII BODY 



Heart 



— Carapace 



Part III 



Pericardia 

 cavity 



Muscle 



Branchiae 

 arthro 



Arteries 

 sternal 



ral sinus. 

 d in open 

 ce. 



FEMALE CRAYFISH 



Cross section of the body through the heart-, 

 arrows indicate the course of blood flow. 



Fig. 13.4. Respiratory organs of invertebrates. A, Blood gills of crayfish visible 

 in a cross section through the thorax. After its passage from the heart and through 

 various arteries the blood flows free in the tissues, through the sternal sinus, 

 thence throughout the vessels of the gills and back to the heart. (After Storer: 

 General Zoology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1951.) 



Gills. Gills are the characteristic respiratory organs of aquatic animals. The 

 majority of them are outgrowths of the body wall that contain circulating 

 blood (Fig. 13.5). Those of immature aquatic insects contain air tubes (Fig. 

 13.4B). Blood gills are most commonly near the head, associated with the 

 pharynx as in fishes. Experiments on the larvae of Salamandra showed 

 the responsiveness of the gills to their environment. When these larvae were 

 kept in highly oxygenated water, their gills grew very slowly, while in control 

 animals kept in water poor in oxygen the gills grew very large, as if striving to 

 satisfy the body's demand for oxygen (Fig. 13.5). In spite of this attempt at 

 compensation the metabolism was reduced and the growth of the body was 

 slowed. 



The gills of fishes are in the sides, apparently of the mouth cavity, actually 

 of the pharynx. When a fish breathes it opens its mouth and the fleshy skin 



