MORPHOLOGY OF ANIMALS 



FIG. 10. Cross section of arm of Starfish ; ep, epidermis ; d, dermis containing calcareous 

 plates ipl) and spines {sp) ; c, coelom ; gp, gastric pockets, lined with endoderm (en); rn, 

 radial nerve, lying in epidermis ; tf, tube foot, containing a water tube which communicates 

 with an ampulla (am) ; s, sucking disk at end of foot. 



4. Alimentary System 



With the exception of a few internal parasites which 

 absorb their food in a digested condition from the bodies 

 of their hosts, some form of digestive system is present 

 in all animals. 



Digestion is the process of rendering insoluble foods 

 soluble and dialyzable. One of the distinguishing charac- 

 teristics of animals is that they, unlike plants, take in, 

 through a mouth opening, solid food, much of which is in 

 an insoluble condition. This process is called ingestion. 

 By the process of digestion some of this insoluble food 

 is rendered soluble, and hence capable of diffusing to all 

 parts of the organism, where by a process known as assimi- 

 lation some of it is built up into the substance of the proto- 



C 34 !] 



