120 



PRINCIPLES OF ANIMAL BIOLOGY 



intestine which is used as a common passage for undigested materials 

 from the aUmcntary tract, for urine, and for reproductive cells from the 

 urinogenital system. It occurs in a few mammals and in most other 

 vertebrates. The large intestine opens to the exterior by means of the 

 anus. 



Conditions Necessitating Respiratory System. — Special organs for 

 respiration apparently arose much later than organs of digestion since 

 they do not occur in the coelenterates, rotifers, flatworms or roundworms, 

 while digestive organs are present in these groups. Most Annelida 



mu 



lint 



Py pa 

 Smtnf. 



Fig. 84. — Dissection of the frog, side view, a, anus; au, auricle of heart; cbd, common 

 bile duct; cer, cerebrum; cl, cloaca; crb, cerebellum; du, duodenum; fb, fat bodies; ubl, 

 gall bladder; A;, kidney; la, larynx; li, liver; I. int, large intestine; l.opt, optic lobe; lu, lung; 

 m, mouth; med, medulla oblongata; mu, muscle; n, nasal aperture; pa, pancreas; py, pylorus; 

 s, spleen; sm. int, small intestine; spc, spinal cord; st, stomach; t, tongue; ts, testis; ub, 

 urinary bladder; ur, ureter; v, ventricle of heart; vd, vasa efferentia; vs, vesicula seminalis. 

 {Modified from Parker and HasweU's Textbook of Zoology, after Marshall.) 



(true worms) have no special respiratory system, nor do they require 

 one since the moist mucous surfaces of their bodies serve the purpose of a 

 membrane through which gaseous exchange may take place. In general 

 it may be said that a respiratory system has developed pari passu with 

 the development of a circulatory system. In many small animals living 

 in water or in moist environments, respiration may be carried on through 

 the general body surface, and in such animals the diffusion and osmosis 

 of oxygen and digested food through the tissues is sufficient to care for 

 the needs of the cells farthest from the sources of these substances. With 

 increased mass of body, however, and with the decrease of surface in 



