MORPHOLOGY OF 11 Id HER TYPES 



121 



proportion to mass, ordinary processes of diffusion and osmosis are not 

 sufficient to meet the needs of the more distant cells. The difficulties 

 arising out of the disproportion of mass to surface have been obviated by 

 the development of a circulatory system, by which a fluid is taken round 

 and round in a course leading past the digestive organs and in contact 

 with moist membranes through which gaseous exchange may take place. 

 In the presence of respiratory and circulatory systems the mass of the 

 organism may be greatly increased. In 

 fact, but for the development of these two 

 systems the size of animals would of 

 necessity have remained small and the 

 efficiency of the organism limited. 



Types of Respiratory System. — Respi- 

 ratory organs are of three general types, 

 namely, trachece, gills, and lungs. 



Tracheae. In most insects at certain 

 points on the body are openings called 

 spiracles to which are connected tubes, 

 the trachece, which by their branches con- 

 duct air to all parts of the body. This 

 type of respiratory system, which is illus- 

 trated in Fig. 85, is not dependent upon 

 the circulation of an oxygen -bearing liquid 

 to carry on the process of respiration, since 

 every tissue is directly supplied with minute 

 air passages through whose membranes 

 gaseous exchange may take place. 



Gills. Gills are probably an earlier 



and more fundamental type of respiratory 



mechanism than tracheae since they occur 



in more primitive animals. Gills are to -r, «- m , , 



. . Fig. 85. — Tracheal system of an 



be considered as filamentous evaginations insect, a, antenna; b, brain; I, leg; 



of an external mucous surface, or in fishes "• "^rve cord; p, palpus; « spiracle; 



s<, spiracular branch; ^ chief tracheal 

 Ot the endodermal layer of the pharynx, trunk; v, ventral branch; vs, visceral 

 Each filament is supplied with blood hranch.^^^lJ^romFolsom'sEntornoloov, 



vessels which form a part of the circula- 

 tory system. Through the thin moist membranous walls of the gills 

 gaseous exchange readily takes place. Gills occur only in aquatic animals 

 or in terrestrial animals which have a method of keeping the gills moist. 

 Examples of animals making use of gills are many polychaete worms, 

 many mollusks, the Crustacea, all fishes, and most amphibians in the 

 larval state. 



In vertebrates the gills are attached to cartilaginous or bony arches 

 located in the region of the pharynx. There are slits between the gill 



