4 8 



AN INTRODUCTION TO ZOOLOGY 



directly with the alimentary canal, but more than this, they arise 

 in the developmental stages of the animal as actual outgrowths of 

 the canal, and so may be regarded as parts of it separated off and 

 modified for special functions. 



The front part of the buccal cavity is lined by a layer of 

 very thin, flat, approximately polygonal cells, i.e. a squamous 

 epithelium. In reality it is a stratified epithelium of many layers 

 with squamous cells on the outside. Each cell is a very thin scale 

 composed of coarsely granular protoplasm with a 

 distinct granular nucleus. The various cells are 

 held together by a certain amount of intercellular 

 connective substance.* A number of glands open 

 on the roof of the mouth just behind the pre- 

 maxillae, and they secrete a mucous-like substance, 

 whose principal function is in connection with the 

 capture of food. As the tongue is slung out of the 

 mouth it wipes this region, and so becomes covered 

 with sticky secretion to which the prey adheres. 

 No glands comparable with those producing the 

 saliva in the higher animals are present in the 

 frog, and the digestion does not commence until 

 the food has reached the stomach. The teeth, too, 



the mouth. 

 Magnified 260 

 diameters. 

 From Quain. 



FIG. 15. Over- 

 lapping squam- 

 ous cells from 

 the inside of are not used for mastication, but only for holding 



the prey during the process of swallowing. 



Further back, in the region of the pharynx, the 

 mouth is lined by highly specialised cells. Each 

 cell is roughly cone-shaped, with its inner pointed 

 end resting upon a basement membrane, and possesses a large 

 distinct nucleus within its finely granular protoplasm. The side 

 of it turned towards the mouth is covered with a coating of very 

 fine hair-like processes, termed cilia, capable of executing quick 

 whip-like movements. The cilia are minute projections of highly- 

 modified protoplasm, and lash sharply in one direction and then 

 return more slowly to their initial position. Their structure and 

 mode of action may readily be studied by examining a scraping from 

 the pharynx of a freshly killed frog.j At the edge of the cell, just 

 beneath the bases of the cilia, are a number of small refractive 

 granules, and in suitably prepared material it appears as if each 

 granule bears a cilium, and is also continued as a thin fibril into the 



* More typical squamous cells may be obtained by gently scraping the 

 inside of the human cheek with the handle of a scalpel and their structure 

 examined by mounting the scraping in a drop of salt solution on a slide. 



f Very good examples of such cells may also be obtained by teasing up 

 portions of the gill of the mussel in salt solution. 



