ALIMENTARY TISSUES 28 1 



up to become a suitable object for the digestive juices to act upon. It 

 may also be crushed or ground after being partly digested. This work 

 is called mastication. It must then be acted upon by the tissues differ- 

 entiated to prepare it for absorption. This process we shall designate 

 as digestion; when digested the food is ready for other tissues whose 

 surfaces will absorb it. This is absorption. All of these processes, 

 except perhaps mastication, are differentiations of the simple alimentary 

 processes that take place in the Protozoa and in the alimentary epithe- 

 lium of the sponges and to be a degree in some higher forms. 



All masticating structures may be considered as mechanical, diges- 

 tive structures. In all forms where an enteron is present, the muscular 

 layer of the alimentary tube must be considered to be a masticating tissue 

 as well as a structure for moving the food through the alimentary canal. 

 As it becomes more highly developed, cilia become less frequent. In the 

 Coelenterata the muscular wall of the enteron is composed of the layer 

 of striped muscle given off by the ectodermal cells. In all these forms 

 it -is of very low development and probably secondarily concerned with 

 alimentation. In the platyhelminthes it has become a definite layer 

 of non-striped muscle fibers closely applied to the basement membrane 

 of the endodermal epithelium. It is here clearly concerned primarily 

 with alimentation. This structural feature persists throughout all 

 higher forms. The muscular layer of the alimentary tube becomes in- 

 tensified in various regions to act as specialized masticating structures. 

 The " gizzards " of annelids and vertebrates, and the stomachs of 

 mammals and some crustaceans all present examples of such muscular 

 development. The epithelium of these regions becomes more compact 

 to serve as protective rather than secreting or absorbing structures. 



In certain fishes the action of the muscular wall of the stomach is 

 aided by calcareous structures laid down by the epithelium of the enteron. 

 The most common and most highly specialized masticating structures 

 arise from the surface of the stomodaeum. The honey-stomach of the 

 bee and stomach of the lobster are the dilated posterior parts of the 

 stomodaeum that have developed chitinous masticating structures which 

 operate together with the special muscular supply of this region, The 

 radula of a snail is a cuticle of chitin formed by the epithelium of the 

 mouth and oesophagus. The teeth, most common of all masticating 

 structures, are but highly specialized integumentary structures. The 

 transition from a scale to a tooth is easily seen in the region of the mouth 

 and on the jaw of an elasmobranch. 



Certain glands constitute a second class of accessory alimentary 

 tissues. These are more or less complex in their structure and may arise 

 from any region of the alimentary canal, but they always open into the 

 lumen of this canal. Such accessory structures vary with the character 



