CHAPTER IV. 



DIGESTION IN THE INVERTEBRATA. 



Digestion in Particular. 



In the present chapter we describe in detail the physiology of 

 the digestive function in certain selected types of all the 

 more important branches of the Invertchrata. 



The Protozoa. 



The Protozoa having no differentiated parts, the cell itself 

 performs, among other functions, that of digestion. This 

 function is diffuse in the lower animals, and only becomes 

 specialised or differentiated as we ascend in the zoological 

 scale. 



The Porifera and Ccelenterata. 



Araongr the animals with cellular differentiation — the 

 Porifera and the Ccelenterata — the internal cavity of the body 

 (morphologically identical with the alimentary canal, and not 

 with the somatic or body cavity of other animals) has the 

 function of a digestive cavity. 



Concerning the function of digestion in Hydra fnsca, 

 Dr. Greenwood* has recently come to the following con- 

 clusions : (a) the ingestion of solids is performed by slow 

 advance over the prey of lip-like projections of the animal's 

 substance. Entomostracea, Nais, beetle larvce, and raw meat 

 prove the most acceptable food ; innutritions matter does 

 not act as a stimulus to digestion, {li) The digestion of 



* Journal of Phi/siolcgi/, vol. 9, 317. 



