604 



THE POPULAR SCIENCE MONTHLY. 



While the whole digestive tract serves the purpose of a reservoir, 

 the special reservoirs have indeed a digestive function, serving to de- 

 lay the food, that it may be acted upon for a sufficient time by the 

 chemical fluids. Thus the crop of a bird secretes a fluid which softens 

 and prepares the hard grain for subsequent trituration and digestion. 



Organs of Chemical Digestion. As the organ of digestion 

 proper is the one most nearly universal, it consequently affords the 



finest example of specialization and de- 

 velopment. From the improvised cav- 

 ity of the amoeba, there is a steady 

 progress by minute steps to the com- 

 plex apparatus of the mammals. Di- 

 gestion is not more perfect, however, 

 in the latter than in the former. The 

 simple nutritive act of the amoeba is as 

 perfect for itself as the differentiated 

 process of the highest animals is for 

 them. In the lowest animals, the func- 

 tion is single, and so simple that no 

 special organ is necessary. As we rise 

 in the animal scale, the function is di- 

 vided into secondary functions, which 

 require for their performance a corre- 

 sponding number of special organs. 

 Indeed, the complex functions of pre- 

 hension, mastication, digestion, and cir- 

 culation are only subdivisions of nutri- 

 tion which begins in the lowest life as 

 a single act. The present purpose, hoAv- 

 ever, is not to trace the evolution of 

 specialization of the digestive function 

 further than to illustrate its general 

 principles and methods, and present 

 some of its peculiar and interesting 

 features. 



Fig. 3. Taenia solium, or Solitary The tape-WOrm has no digestive or- 



Worm : a, head, or scolex ; b, tape v a i r ,1 



formed of many individuals, the last g ans whatever, having no use for them. 

 S f nd w ef &TK!^SS^SS A 'obber subsisting on the labors of 



^"X^SSffi^'^Sfta^: its victim > Jt takes food the ^me 



manner as a plant, by absorption from 



the outside. This is also the case with many lower protozoa. 



The digestion of the amoeba is only one remove higher than that 

 of the tape-worm with no permanent organs, but extemporizing a 

 stomach from the skin as required. A step higher still we find the 

 hydra, with a permanent body cavity serving the purpose of a stom- 

 ach. But it is not distinctively a stomach, as it is the common organ 



